Update CI to minify PNGs

[aneshodza]

As seen in this CI run I had the issue that GitHub would just leave bigger images blank. That would look as follows: Minifying the images fixed that issue, so I thought maybe we add it to the CI. For the moment its on level two, but it goes all the way from zero to seven. My 2nd idea was, that the default should be level zero and then adding a notice in the readme, that minifying images may be necessary if there are blank images like that one.