I mean, there are more than enough free online tools to resize images and .gifs, so to say that "not everyone has access" isn't quite correct, and even a no-frills editor shouldn't cut the quality enough that it'd even be noticeable when sized down to avatar proportions. An automatic avatar resizing within vBulletin itself would likely compress and distort the image far worse than literally any resizing tool you'd find online.
I can safely say this isn't something we're currently looking into and doubt we'd ever take the time to see if it'd even be possible. It's very simple to resize an image and we have faith that the vast majority of TBT users can figure it out without much issue.