I will answer to clarify specific issues on the question.
I am going to be picky because I want you to analyze your process, so it becomes more clear to you.
- 1-Adobe photoshop file (.psd)
- 2-High resolution photograph
- 3-1000 px by 1000 px
- 4-Saved as .jpg
- 5-Upload to website as well (file size not an issue)
- 6-Photo can be saved via website and printed
The moment you save an image as JPG (4) does not matter anymore how you worked your photo, Photoshop, PhotoPaint, Photoanything.
2 & 3 are somehow contradictory. A 1000x1000 image is one Megapixel, it is hardly the size to be considered as a wallpaper for old monitors; a simple wallpaper for an HD monitor is around 2 Mpx. Of course, depends on what was the camera producing that image, for example thermal cameras normally has lower resolution and in that case 1Mpx image could be a High res.
Upload to the website as well... This is, are you uploading your 1Mpx image to the website? Therefore this 1Mpx image is going to be printed? You are going to print a low-resolution image.
"Print: 300dpi is standard, sometimes 150 is acceptable but never lower, you may go higher for some situations."
That statement is nonsense, because Print resolution, (expressed as ppi) depends on the viewing distance, original resolution, intended usage, etc. A billboard can be viewed at 100 mts, and can be printed for example at 1 ppi. A banner for a store could be fine at 25ppi, etc.
Website 2: 72 DPI
Well the link you consulted has this right... For a website the 72 ppi is TOTALLY IRRELEVANT. What matters there is that you have 1000 x 1000px period. So forget a 72 ppi version.
The question is:
Do you want to control what the physical size of the print on the anonymous user's home is going to be?
Then you care about the ppi (but in this case providing a JPG is not a good idea, you should use a PDF file)
Do you want the user to print a 1000px photo at 300ppi? Fine. The image will be 3 inches tall.
And guess what, the 72 DPI image looks bigger. How???
No and no. In your second reference, it clearly shows on the webpage 3 identical kitty photos.
The PPI info embedded on a jpg file is just a SMALL recommendation to the user's printer, that can be overrun, overwrite, forgotten into oblivion if the user turn on the fit to page checkbox for example.
If the user leaves some mysterious checkbox on his printer's window "use the ppi info on the photo" then, less ppi will be used every inch.
Kids eating candies
"Kids, how many candies each one will eat? I only have 1000 candies"
a) "We will eat 300 candies each one" So only 3 kids will eat candies. A small table will do.
b) "We will eat 100 candies each" So 10 kids can now eat candies and you need a bigger table for them to be.
3) "Kids will eat some amount of candies, depending on how many kids come to the party" Then the operation depends on how many kids actually you invited. If you invited 20, or 50 then the amount of candies will be different, and the size of the table to accommodate them will change.