Malicious Code Detected error

What's up with that shit? I hang out on /co/ a lot, and sometimes when I storytime a comic I won't be able to post a page because of the malicious code detected error and I'll have to change a pixel or rename it or save it in a different format or something. One time I wasn't even posting pages I'd downloaded, the comic was in a PDF so I had to do this convoluted copy/paste/save thing, and I got the malicious code error on one of the pages, even though the page had been created exactly like every other page. What the heck, man?

You probably have a virus now. Remove the suspicious (image) files from your PC, clear your cache, change your IP if necessary.

This mostly happens with GIFs. Guessing there's a certain string in them that was used for things like 4chan.js but can also occur `naturally'.
If this is the case, use JPEGs or PNGs.