I had a huge problem with getting an HTTPerror 403 while uploading images using the upload() command in Flash 8. This is the solution off of the Macromedia website...
Any http errors such as 403, 406 or where a file upload goes to a 100% but doesn't show up on the server is a result of the mod_security. Mod_sercurity blocks the post request of flash because it is missing a ":". To correct this problem create or find the .htaccess file in your root directory then type in:
SecFilterEngine Off
SecFilterScanPOST Off
that should correct your problem