If we encounter error 403 while uploading images in Joomla 1.5, try adding these 2 lines in .htaccess:
SecFilterEngine Off
SecFilterScanPOST Off
The fix works for me.
References: http://www.dart-creations.com/the-joomla-blog/blog/media-manager-upload-failed-403-error.html
Posted by singchyun