Pretty much as it says in the title. If you add a map board and select the image, save and close, then you edit the module, go into the map board, and select instead a different image with a different name, the compressed file will grow considerably (in fact ad infinitum if you keep changing the image with one of a different name). It seems the previous images remain in the compressed file.
The obvious way to avoid this is to make sure any replacement image has the same name as the original. However, if you made the mistake of using a different name, it's too late as now both images are in the compressed file and can't be extracted from it (as far as I can see). The workaround is to create 1kb images of all those unwanted ones and attach them in turn to the map board one after the other, and then re- attach the one you actually want. Thought you might want to know about this for a fix...
