Image Block adds a toggle button that conditionally blocks/allows loading of images on webpages. Blocking images results in faster browsing, especially helpful on handheld devices or slower connections like GSM/GPRS/EDGE dial-up etc.
Here is a little more about Image Block from the developers:
Recently in a review one user (thanks dellvin!) pointed out that Image Block is quite useful when you visit websites which show dirty and adult advertisements, while also containing useful information. If you are not sure of what you are about to see on a webpage, just block images to avoid possible embarrassment.
It can also be used at work where your boss doesn’t allow you to surf the web or social networking websites!! They look very different (hard to see which site you are viewing) with image blocking on. Want to give it a shot? You can download Image Block from the Firefox Add-ons website.