Does downloading an image off a website, when Wi-Fi is strong, result in a higher-quality image on your device?
Does downloading an image off a website when WiFi is strong result in a higher quality image on your device?
Signal quality does not usually affect the transmission of data that is sent, though it might result in incomplete transmission. However...
Websites often send different data to mobile vs desktop.
Some sites do use scripting to send different data depending on link quality and speed. This is especially common on video streaming sites.
Some browsers use proxy servers to accelerate transmission speed by compressing data. This may include lossy recompression and resizing of images.
Some sites may display a more highly compressed image than they send for download. This is often the case when a zoom function is present.
The other answers point out that the quality of digital images does not deteriorate during transfer. It is worth pointing out that many mobile data plans these days, however, transcode at least movies and deliver them with lower quality than the original if they are transferred via a channel accessible to the service provider. That can either mean non-encrypted transfer or transfer of an encrypted channel for which the service provider has a caching arrangement with the content provider (typical for platforms like Netflix). Depending on the plan and its provider, non-encrypted images on web sites may be equally afflicted.
With such a data plan, being connected via WiFi would increase the typical image quality. The result would not vary depending on the quality of the WiFi connection but on whether the phone uses it at all instead of relying on the possibly recoding mobile data plan.
So in principle the quality of images does not depend on the quality of the channel used for getting them but on whether somebody messes with your data. And these days, they may.
Even though the other answer here are already very good, allow me to give a different perspective:
In general, no.
When you download an image in your browser, it is very likely that this file (the image) will be downloaded through the Transfer Control Protocol (TCP). TCP will split up your image in little packets, and send each of those separately over the internet. Each packet is accompanied by a checksum. This checksum is the result of a certain mathematical operation on the packet. Once received, the checksum is calculated again by the receiver. Both checksums should be the same. If not, that means at least one bit has changed or information was left behind. In this case the receiver will request to send that particular packet again. Once all packets have been received, the downloaded file will be identical to the original on the server you downloaded it from.
However, you're streaming a movie, or viewing images over a webstream, chances are that it is not TCP, but UDP(User Datagram Protocol). UDP does NOT do the same checks as TCP does. it is called "fire and forget". If packets get lost or damaged, they will not be repaired.
No, computers communicate on a bit-perfect level -- even a slight change during transmission might completely corrupt the data, and is protected against using checksums (and retries in case of errors).
However, it's possible to have "progressive loading" of images, where e.g. a website might first display lower-resolution images as a quick placeholder (to reduce the perceived load speed), replaced by the higher-resolution images once they have been downloaded. If your connection is really slow and/or unreliable, it is possible that you would get stuck with the lower-resolution placeholder image.
Edit: To clarify, it's possible that the quality may be reduced due to a number of factors, but it's because the web page (or a proxy) chooses so -- not because of transmission over wifi would deteriorate the quality.
The other answers are correct (no loss in quality of images over WiFi), but I just want to point out:
You may have seen that the quality of streaming(!) videos seems to degrade on 'weak' connections. This is however not caused by some of the information being lost during transmission(*): Most video streaming servers today maintain multiple copies of the same video, each compressed with a different compression ratio. Higher compression ratios yield smaller files, but because the compression is lossy the quality of those smaller files is worse.
Now, what happens while you stream a video is that the streaming server monitors the 'quality' of your connection, i.e. the data rate of the connection. When the server detects that your connection has become slower, e.g. because you moved your device and the WiFi signal becomes marginal, it will more or less seamlessly switch to a lower bitrate stream of the same video, sacrificing image quality for being able to continue watching the video instead of having the (high quality) video pause repeatedly while the next few seconds of the stream are downloaded.
This works the other way around too: When the server detects that your connection's bandwidth increased, it may switch to a higher quality version of the stream trying to always give you the best image quality possible with your current bandwidth.
*) Actually, it is indirectly caused by loss of data packets: When the signal becomes weak, random data errors become more frequent. However, these data errors are detected by the hardware and 'broken' packets will usually be sent again a number of times until the packet is received correctly; this however means that each packet needs (on average) to be transmitted over the WiFi link more than once, e.g. three times, which means that the achieved data rate (i.e. packets successfully transferred per second) drops to e.g. 1/3.
To add a bit more flavor to these other answers, here's several copies of a picture that I just took:
Unikitty 1 - Resized ?x400px, no change in quality
Unikitty 2 - Resized ?x400px Quality 50%
Unikitty 3 - Resized ?x200px Quality 50%
Unikitty 4 - Resized ?x200px Quality 5%
On my computer I have stripped the metadata with
exiftool --all=, and computed checksums of the images:
$ sha256sum unikitty*.jpg 2ead7f2b1c5453f2a80da46f131c95be3423fc92ef8bce43b95fac3ee483d1b3 unikitty.jpg 42358f2f2447568fedc31a03575e8850406f89ff0e5d5a6e2d15c9e492205279 unikitty1.jpg 5c05365cb4aea2f100da471cdee85e5a3942509ab09cf3aa646fddb6962bd95a unikitty2.jpg e48c7158bbb9476cacdc80308832d76384e3016fb4dd2afd4333970781b367d5 unikitty3.jpg 986f8c64745fb61196df662c5e9a7c799ed370749f3dc367f269162bfa413800 unikitty4.jpg
However, when I download Unikitty #4, the hash has changed:
That means that the file has definitely changed. Exactly how, I'm not sure. Probably Imgur doing something to resize it.
If you want to be positive that the picture that you're getting is the picture that someone else is also looking at, you must have a checksum to verify that none of the bits got mucked up in transit. For the most part, the Internet does that (TCP/IP tells how to make sure that when you send information it all gets to the place that it's supposed to go, and if it doesn't get there then it has to complain loudly. Most of the Internet uses TCP/IP because of that) for you. However, as others have mentioned, there are a myriad of ways that quality of the image can change.
If you download Unikitty 4 and get the same SHA-256 sum that I did, you can know with almost complete certainty that the picture that you're looking at and the picture that I'm looking at are 100% identical.
Though then you have to worry if our monitors are color-calibrated the same, and if the ambient lighting is affecting anything, or...
It is possible that you are viewing the images as they are being downloaded (photos stored on cloud app like Google Photos) or it is fully downloaded but the software you are using to view the image hasn't completed rendering - so you are seeing an image in progressive rendering mode.
To answer your question, No the quality (i presume you meant the speed) of the network doesn't affect the quality of an image.
No. Digital files, contrary to their analog counterparts, are always perfect copies, so the image either fails to download, or is downloaded perfectly, every single time. Same thing applies to transfering them between mediums (SD to disk, pendrive, CD, ...) or creating copies.
Corrupt copies are possible, albeit very improbable, on some cases, but they are not "lower quality", but completely wrong (ie: only half the image and such).