Can't insert pictures into posts

I updated WordPress 2 days ago, but didn’t do anything with my blog (http://tramplingrose.com) until tonight, when I discovered that while trying to post, I’m unable to insert pictures from my media library into my post. This is the message I get:

An appropriate representation of the requested resource /wp-admin/media-upload.php could not be found on this server.

Additionally, a 404 Not Found error was encountered while trying to use an ErrorDocument to handle the request.

I’ve re-installed 3.1.1, cleared my cache a few times, deactivated and reactivated & updated all my plug-ins and I have no idea what to do. I should also mention that I’m virtually ignorant when it comes to fixing problems like this–I’m just a blogger who kind of freaks out when things break, so any help with this is MOST appreciated!