Black pixels in (recompressed?) images turn white.

This image, pasted into a conversation, shows white pixels that were originally black :

@hotzenplotz3000 > @PatrickvL these white pixels only appear only on gitter... My screenshot looks fine

This might be a recompression-artifact of some kind, but is highly misleading in any case. (I report it as I started to doubt a color-conversion algorithm I've been working on that handles the display of these images - hence my confusion when seeing this error).

Please find out what's causing this, and fix it

Thanks in advance.

@PatrickvL Can you attach the original source of the image so we can test with it?

moved to webapp-temp-issues-for-move#96 (closed)