[DustyPorViva]

With lineart, it's okay to save with JPEG, seeing as though there is nothing to critique but the lines, which don't really become distorted.
But with pixel art, mainly, JPEG ruins the quality, and pixel art, every pixel counts, and JPEG can really alter that.

With larger pictures, or photos it's okay, but with pixel art, I wouldn't recommend JPEG -- better with GIF or PNG.