Um... idont knwo why you think image data in a DLL is going to be any smaller then image data in a JAR file.

In fact, Im not entierly sure how you itnend to PUT an image in a dll since a dll is a code file..
But in anya case there is no need to put your images in the JAR. You can simply serve them to client with an HTTP server and laod them as needed, if you want.  You could even then cache them locally and not update them unless the date changes.
(Yould have to write the caching code yourself. Java does eveything else for you.)