It gets even weirder.
It seems that it's only broken since a certain date, because I have no problem displaying the same picture Windwalker tries to use.
Also this problem isn't just with images. Look at the signature of elafreat for example: http://www.scirra.com/forum/translating-800-words-from-en-to-ar-for-5_topic76816_post459910.html?KW=#459910
There's a url tag not working...
EDIT: Sorry, I just noticed that the url tag was used wrong. So the elafreat issue is not related!
It seems that bbcodes are not properly translated. Here's a comparison between my signature and thehen's:
<img src="https://dl.dropboxusercontent.com/u/11182740/pictures/html_code.png" border="0">