ImageTweak
ImageTweak is an add-on for Firefox that allows you customize the way that you view images. You can choose to view images centered in your browser window, you can change the background color that your image is placed against, and you can easily and instantly zoom in and out of images.
To access ImageTweak's options got to Tools > Add-ons > Extensions > ImageTweak > Options. Your options will be applied as soon as you exit the ImageTweak options window and refresh the page that your image is displayed on.
