Personally speaking, I'm much more likely to approve a highres/absurdres post if it's saved in jpeg format, and if it was saved at a high-enough quality level to not notice any degredation.
Large images uploaded in PNG format, to me, just feel like overkill here. Though I do think it makes sense on imouto, since it's supposed to be an archive for high-quality scans (so using a losseless compression format is important).
