Oh, that's annoying, isn't it?
Have you tried uploading a full-size photo to your site, then once it's there, downloading it again to see what size it is?
I've just done so.
But if you're dealing with a glare problem of your own, please start your own thread with it. These fixes are quite specific to each photo.
I'll answer this part. No. You only ever need your resolution to be such that the pixels are small enough to reproduce the smallest detail of your wife's brush work. And on canvas, this is not very small at all.