This can actually be useful if you want to make sure your interface works with images turned off.
Some people, especially mobile users concerned with bandwidth, might have images turned off but CSS turned on. I had icons for some buttons, sprited and set as CSS backgrounds. Inside the same element I put an img tag with a minimal gif and an alt text with a fallback (a text label, or a single unicode character). The invisible image would be shown for users images, and the alt text for users without images.
Some people, especially mobile users concerned with bandwidth, might have images turned off but CSS turned on. I had icons for some buttons, sprited and set as CSS backgrounds. Inside the same element I put an img tag with a minimal gif and an alt text with a fallback (a text label, or a single unicode character). The invisible image would be shown for users images, and the alt text for users without images.