There is a trick with which someone helped me.
Even then, the maps were in png form and each over 20MB, some very much large than that such as with Brügner's Bautzen was 143MB.
Then converted to jpg files and they were still very large. For example Brügner's Bautzen ended up about 15MB.
Shall have to remember how we did this and shall post again.
Think it had to do with a de-zoomify program. - R