At first I saved the .jpg image with a quality of 100% - but in order to attach the image to the forum message, i needed to re-save the image with 92% - as I needed to get below 256KB.
Funny thing is that I only see the error in browsers (and the Thunderbird email client), that probably rely on the same .png library. Just tested Google Chrome - and the same error appears. One could get the idea, that it's actually just one single .png library, that's having trouble with the colours of the files.