External resources ( this includes LINKED .js, .css, images, anything with src='') gets cached As long as the user doesn't the image is reused and not d/l'd.
Cache stores a resource file from that particular http address ( your image) under a unique temp name. A neat benefit of this, if you use a common resource, it may already be in the users cache from a previous visit or a visit to another site that uses the same resource. This is more likely to happen with .scripts and frameworks than images... but it's nice to know.
So the they there is the SRC, the same image being loaded from different URLs really counts as different images.