So you want to add moon maps to Marble. This is great since we want to see lots of maps in Marble where the user can browse through space and time. There are lots of maps that are under the public domain.
You'll find a lot of maps at sites like:
Before start creating a map, check if its also created at Marble Maps. Normally these type of maps or rather images, are uploaded in several projections, to create our map, we need a simple cylindrical version. If the map that you want it's not available in a simple cylindrical version, you can follow the steps that are explained at Historical Maps
As an example we start off with this nice map of Iapetus from the Cassini and Voyager missions:
This map has a resolution of 2880 × 5760 which gives us good details, and marble can create the tiles automatically, because it's under 60000x120000.
We create a directory "hondius1595" and save our file "1595_Vera_Totius_Hondius.jpg" there.