Thanks so much to pumpkinslayer. Adding the following to the top of my .htaccess file solved the image upload problem for me:
<IfModule mod_security.c>
<Files async-upload.php>
SecFilterEngine Off
SecFilterScanPOST Off
</Files>
</IfModule>
Never had any problem with images before upgrading to 2.5.1, but as of that version image uploads would not work at all (and my host won’t even let me change permissions to 777 on a directory; that’s no answer). Apparently it’s this mod security stuff that causes the problems.
