After upgrading to WP2.6.1 I realised that I get a HTTP Error message whenever I try to user the image/media uploader. A fix is simple - just create a .htaccess file and paste this piece of code in it:
<IfModule mod_security.c> <Files async-upload.php> SecFilterEngine Off SecFilterScanPOST Off </Files> </IfModule>
And then upload it to your wp-admin folder. This will most probably fix your problem. However, if it doesn't, it means a bigger problem that is lurking underneath. For that, I couldn't help anymore but to redirect you to Otto's comprehensive list of possible fixes for the uploader bugs.