Dafault image for each category

Currently, to have a post on my homepage show an image, I have to select a featured image on the side panel, AND provide the url in a custom field and post it into the actual post, in order for it to show on the main page, instead of simply a grey box or an "x" box. I'd like to be able to select a different custom image for each category to show on the main page, in the event that a poster does not properly post featured images...so I don't have to EVER see those "x" boxes. Also, just started getting this, when I try to post: Internal Server Error The server encountered an internal error or misconfiguration and was unable to complete your request. Please contact the server administrator, and inform them of the time the error occurred, and anything you might have done that may have caused the error. More information about this error may be available in the server error log. Additionally, a 500 Internal Server Error error was encountered while trying to use an ErrorDocument to handle the request. -------------------------------------------------------------------------------- Apache/2.2.23 (Unix) mod_ssl/2.2.23 OpenSSL/0.9.8e-fips-rhel5 mod_auth_passthrough/2.1 mod_bwlimited/1.4 FrontPage/5.0.2.2635 mod_jk/1.2.35 Server at [login to view URL] Port 80