Hi,
http://www.imageshack.us
It is a very popular site for sharing game screenshots, normal screenshots and photos since you can upload without account etc.
I tried to upload "scr.png" file which is basically a screen shot, Graphic Converter used with "web ready" (no resource) options even with filename checking for web, it says:
Cannot Load Address
Stopped loading <http://www.imageshack.us/>: POSIX error: Invalid argument
No need to say, I tried multiple times and verified it doesn't happen in Safari too.