You have a filter option for images. See upper right hand side of design tool. It won't so much fade the image, but it will change your image in a way you might find useful. (Does not work for all file types). If you want to fade images inside of the design tool, another option is to upload a few low opacity screens to your image library. Then you can lay one of those on top of another image, and get something of a faded effect.
