[Plugin: EWWW Image Optimizer] Some doubts i have about the optimization

First, it’s difficult to say too much with accuracy, because I couldn’t find any place where the author had documented the options that trimage passes to the image compression utilities.

However, here’s what I do know…
Regarding PNGs, EWWW uses optipng, which is a newer fork of pngcrush designed to be more flexible and faster. I’ve chosen an optimization level for optipng somewhere in the middle of what the author of optipng recommends at http://optipng.sourceforge.net/pngtech/optipng.html

advancecomp (advpng) apparently does not do everything on it’s own, and trimage runs either optipng or pngcrush in conjunction with advpng.

All that said, it really depends on what level of optimization trimage uses, and the only way to know, is by testing trimage vs just plain optipng in a few trials (I’ll leave that up to you). The options I use for optipng can be found over on the FAQ. Additionally, in the latest release, I’ve made the level configurable, just beware that higher levels can take considerable time compared to the defaults (even that takes a while as PNGs increase in size).

Now, for JPGs, from what I remember, jpegtran and jpegoptim use pretty much identical optimization, and I suspect you won’t find any difference there. The only extra space to be squeezed out with either program is by stripping out all the metadata (which is off by default in my plugin, but can be turned on via the Settings page).

I’m afraid that’s still awfully vague, and requires testing to prove anything, but I hope it helps.

I released 1.0.5 last night (got a newborn here, so I’m up at odd hours), should produce identical results now between jpegtran and jpegoptim (it did for me). I’d be curious to see what you found out regarding PNGs, and then we can mark this resolved.