I think it'd be a good idea to add a compress/optimize option for PNG images when you upload.
It probably wouldn't be good for stuff originating from pixiv (I suspect it would result in a md5_mismatch), but it seems good when you upload from your computer. Especially when you're uploading high-quality PNGs from game_cgs.
I find myself optimizing them using tools such as OptiPNG and PNGOUT before uploading. And I think optimizing during the uploading process could simplify things more. After all, saving bandwidth is good, right?
What does everyone else think?