There is a bug in the beta version of iceweasel 7, which causes images to be
downloaded, even though image loading is disabled. This can be a problem for
people on metered internet, who have to pay extra for image downloading.

Edit, Preferences, Content, Load images automatically: Unticked, Button: Close

Now visit:

http://www.google.co.uk

Type: giraffe

Images of giraffes appear in the search results. Click on images in the google
toolbar, all of the images appear. None are blocked.

This problem did not occur with version 5, and appears to be a bug introduced
into the new beta.

Possibly related upstream bugs:

Bug 331257 - data: images show up when 'load images' (Tools->Options->content->checkbox) is disabled
Bug 449273 - SVG:display desc (and title?) as text if images are blocked
Bug 1196784 - img tag with srcset attribute bypasses permissions.default.image=2

https://bugzilla.mozilla.org/buglist.cgi?bug_id=331257,449273,1196784