There may be a good reason but your post has no details or clues beyond it doesn't work.
Think over how your machine differs from machines that work. In that you may find why it does that.
I've got a strange problem.. I can upload photos to myspace/facebook, but they won't send as an attachment in an email, nor upload to photobucket, flickr, or imageshack. But they WILL upload onto ImageHost for some reason. While searching around for an answer I've came across quite a few people experiencing this problem but I haven't seen an answer. Any help?
photobucket says: Upload failed for photo.jpg with error: A network error caused your upload to fail. Please try your upload again.
ImageShack.us says: No files uploaded immediately
Flickr: Gets to 100% complete then says: Bonk! None of your files made it.
ImageHost works just fine though, and so does uploading them onto myspace and facebook.