Dynamic change of an image href has been disabled due to potential security issues. There are however a few workarounds to choose from:
1. Embedding the images in the form as hidden objects. They can then be set to visible on initialize or clicking a button. This will affect performance, but if the images are in a compressed format, this can be minimized.
2. Create a Web service interface to grab the image from wherever they are stored, base64 encode it and return it to the form. You would also have to update the bindings for the image field on the form (a one time operation).