In Photoshop Elements, how much is image quality reduced if I re-save a previously saved "small" JPEG as a "large" JPEG?

Question

Back story: I took a picture of a flower in RAW. Pulled it into ACR in PSE 12, moved it to the Photoshop Elements editor, made a few edits, then saved it as a "small" JPEG. When saving a pic in PSE, there is a "JPEG Options" dialog box that comes up and gives you the option to choose the quality of the saved image. You can Choose from a sliding scale from 0 ("small" or low quality) to 12 ("large" or maximum quality). For this one flower pic, I choose a relatively small number b/c I only intended to post online for viewing (4 I think). My wife saw the pic, loved it, and now wants me to blow it up, put it on a canvas, and put it up in the guest bedroom! I go to my favorite printing site, upload the pic for the canvas print, and the site says "can't print at that picture size because your image is too small."

I want to the best quality image for canvas print enlargement.

Question: If I simply re-open the current pic, do no edits, and re-save it in PSE at the "12/large" size setting in the JPEG Options dialog, will the image quality re substantially reduced - making it not good for enlargement? This seems the simplest route to go, but I have a sneaky feeling that re-saving an image that was saved as "small" to a larger size is making a sandwich with "used" bread. Other option is to start with the RAW image again, re-do the same edits and save a new JPEG at the large size setting. Looking for some expert experience/advice on whether there is any real noticable difference btwn these two options in creating a larger sized image suitable for the canvas enlargement.

Answer 1

I'm not familiar with Photoshop Elements, but the "small" and "large" quality options are probably referring to file size, whereas the "can't print at that picture size because your image is too small" message is probably referring to pixel dimensions. To print a large canvas print, you need plenty of pixels, and the JPEG quality setting is not related. In your original editing, did you resize the image downwards? If so, you could resize your edited image upwards again, but you're almost guaranteed to end up with an unsatisfactory image. Better option is to go back to the RAW file and try to replicate your original processing.

Re-saving a JPEG file means applying JPEG compression twice. While it's not disastrous by any means, you should avoid it if you can, even when it results in a larger file size. However, as stated earlier, you're barking up the wrong tree with this approach I think.

Answer 2

If you're lucky, you won't lose any quality, but you certainly won't gain any.

The best thing to do is to re-process the RAW file. Whilst doing so, you can ask your wife if she wants is exactly the same or you could change the framing etc if desired. Bear in mind that canvas wraps can sometimes 'lose' part of the image near the edge, as it is wrapped around the canvas, so take this into account if cropping.