So, after further investigation, Static Images are behaving as they should. After setting the href the first time it can't be changed. It should be possible, however, to change the href of an Image Field as many times as you want, but it doesn't work currently in 7.0.5 or 7.0.7. This is known problem. Unfortunately I don't see any workaround possible for the time being if you need to change the image in a field multiple times. (I suppose you could have multiple image objects, one for each image, and hide/show them dynamically based on which should be shown at the time).