I am not sure this is such a bad idea for two reasons:
Software is commonly released with 'resource files' that contain all of the images, sounds, language/locale specific information for the given client that is needed. If you release your code in a modular fashion you can simply send them an 'update' to give them new resource files
Images are embedded in e-mail now all of the time. Consider Base 64 Encode/Decode with OpenSSL for instance.