Go to about:config, search for this option "permissions.default.image" change it to 2.

Possible values:

1 -- Always load the images

2 -- Never load the images

3 -- Don't load third images

There also are some extensions to create a toolbar button for this, in case you need to do it often. For example:

• Image Block - "Image Block adds a toggle button that conditionally blocks/allows loading of images on webpages."

• QuickJava - adds buttons for turning on and off: images, animated images, JavaScript, Flash, Java... a lot of stuff that may or may not sound interesting to you

See also:

• SettingSanity: https://addons.mozilla.org/firefox/addon/settingsanity/
• Tab Permissions: https://addons.mozilla.org/firefox/addon/tab-permissions/

@Diego Victor: Mine was on 1 for enable images and 2 worked for disable images.