Are you positive about extensions and filetypes? PNGs are problematic as mentioned, I reported this before and tried several solutions (none of them worked, code is simply ignored), later Conejo also tried and gave up since there is an apparent bug with PHP extension, so you may use Force JPEG as a transitional solution if you don't care about quality and retaining transparency. JPEGs already have optimal compression in my opinion: in case of GD since it uses an inferior algorithm which results in a noticeable artefacts lowering it even more is out of the question; with iMagick default quality value is already optimal, don't see a big reason for a change either.
Regards
