Error 500 when uploading pictures

Hi everybody.

When uploading pictures, I get this error in the flash uploader:

Error 500 – Internal server error

An internal server error has occured!
Please try again later.

When uploading with the browser, I get an HTTP error.

I am getting this on two separate sites on my hosting account, one of which is https://delineartdesign.com/blog but neither of which have additional active plug-ins. This blog has been updated recently, but also had the problem in WP 3.0.

Any ideas?
Thanks!!!