Oops, I misread. Not about optimisation but conversion.
That I do with Paint Shop Pro. And Inkscape for svg to png.
Neat for pngs – not an online tool :
http://psydk.org/pngoptimizer
Checks out after virus scans. I saw people recommended RIOT for jpg as an alternative to smushit (which seems to be offline) but that contained 12 trojans.
:-/
Today I discovered this site though :
https://compressor.io/compress
Both optimise in such a way that size is reduced considerably, without any noticeable loss in quality. And Google Pagespeed apparently ‘likes’ the way they compress.