So this is the email I just sent to support@tumblr.com:

Hello. I wanted to send this email to let you know that tumblr’s recent decision to change the dimensions of photosets and photos was, in my opinion, not well thought out. I know you get a lot of negative feedback anytime you guys change anything, but the protests you are currently hearing from the people on tumblr who post photos, photosets, gifs, (ie. the people who make tumblr a place worth visiting) are not just your run-of-the-mill reflexive feet stomping over experiencing the inconvenience of change. The problem is that by making these changes in the way that you have, all the photo/photoset/gif posts made previous to the change now look terrible. Photos and gifs are now blurry and out of focus. On many themes, photos and gifs are now chopped off. This greatly devalues millions of past posts to tumblr. People will be less likely to reblog posts that once looked as their creators intended them to look, and now look blurry and messy. For some creators and artists, this amounts to vandalism of their work. Surely there’s a way to fix this, either by returning photo and photoset posts to their previous dimensions, or by finding a solution that makes at least all posts made previous to the change appear as they were meant to.