ImageTweak for Firefox
ImageTweak is an add-on for Firefox that allows you customize the way that you view individual images.
There are several customization options available including the ability to view images centered in your browser window, setting the background color that your image is placed against, zooming methods, and more.
To access ImageTweak's options, open the Add-ons Manager > 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.
Image zoomed out
Image "Fill View"
Image zoomed in