500 Server Errors can happen when the webserver is short on resources -- too many concurrent connections, memory too low/unavailable.
It can be caused by over-aggressive robots crawling your site, leading to hitting the concurrent connection limit. It also depends on the hosting setup -- shared hosters can establish restrictive limits on connections and memory used by each process for a user. In some cases, all it takes to avoid it is to set memory_limit size down to 64M (default is likely 128M or 256M) for PHP.
In a php.ini try memory_limit = 64M and see if that relieves the issue.
I don't see a current issue with the DualImage link you showed above.. it produces an image, not a 500 Error.
