Adding a clarification to this: I went back and looked at the chat where the Mediawiki guru was explaining the templates she's going to design to me, and I was remembering it wrongly: she was going to design a thumbnail box for the images to appear on articles that had a red border, not a red line over the image. I do apologize for having gotten that wrong earlier, since it's clear that the second proposal is much more upsetting from an artist's standpoint.

msilverstar points out below that if we reduce the size of images that are warned for to 150 by 150 on the article page, that even that might be enough by itself to avoid upsetting users. We can test out what the images look like once the new templates are in place, and see if that's enough by itself.