I had much trouble after updating to 3.5, and I tried everything to get the media library and admin icons to work, and was in the middle of writing a help post to this forum, but then had a 'lightbulb' moment.
It cured my particular problem so I thought I would post here in case it helps anyone else.
Cutting a long story short here...
cPanel Hotlink protection if you have it enabled DISABLE it and everything in you Wp install will then work.
(The hotfix function puts code in your site's htaccess file)
Took me 36 hours to figure that!
Hope it helps someone :-)