500 error when uploading images

everything seems to be fine… except when i try and upload an image… the file goes up fine, but thumb is broken and if you click link to image, it just gives me an internal server error (500). checking FTP client, the file EXISTS as it’s supposed to… just can’t get to it.

feel free to check out sample: http://4theriders.com/riders/blog/2008/02/23/4/ it’s broked.

any help? CHMOD is at 777 like it should be…