Category Icons are stored in /images/, where all the forum images and icons are also stored.
Why are you not trying to upload an image somewhere to a new directory with f.e. the name "testme", uploading an image and calling that image from the web?
http://www.amdforum.se/testme/image.png
If there is no file, I get a redirect to "Sidan du beg?rde kan tyv?rr inte hittas. " But I don't get this error calling the xperience images.
Please check also if there is no custom rewrite rule in place. If you are using the rewrite rule form KURTZ, disable it.