FF had a setting, to disable images - for a sites list. This option (button which opens sites list to disable images) is removed from "Contents" tab of options, in FF 24 for Win. I cannot edit it now. How to edit it (or reset it)?
For individual sites it's possible to reset/set the permissions when visiting the site -- right-click > View Page Info > Permissions: Load Images.
Another way would be to enter Permissions Manager, select the site on the left, and Forget About This Site on the right (though image options aren't listed as of now, it works). Alternatively via History Library (Ctrl-Shift-H), search for, and right-click the site on the right-side pane > Forget About This Site. Please note that both these methods also resets all the settings for the particular site to the defaults as shown when selecting All Sites (on the left) in the Permissions Manager.
It's also possible to reset all permissions (including images) for all sites to the defaults by deleting permissions.sqlite in the Profile folder after exiting Firefox.