Unfortunately there is no way to do this. Themes do have 'Image' controls but they are static and sent from the server to the client initially when the client loads. To have a dynamic type of image control would require that new images be downloaded to the client on demand.
It's a good idea and could probably used in other situations as well. I'll add it to the list, but it may be a while before I get to it. Also, currently on bitmap (BMP) files are supported in Themes. I know CDJ supports JPEG formats. It would be nice to support other image types like JPEG, but it would probably be a fair amount of work to do so.
