Lossless image compression

When dealing with a previous issue ImgBot automatically opened a PR against my fork on GitHub to do lossless image compression: https://github.com/MichaIng/phpbb/pull/1

I had a quick look through it, especially the ones with high compression ratio and the animated GIFs and indeed it did a good job.

For forum software it is really a minor thing as forums usually have no representation task like front websites where the first impression may count. When visiting the forums once, the images are in cache and those 190 KiB additional space really should not matter.

However, if some perfectionist has interest, I can open the PR against the upstream repo. Otherwise close the issue .