We'd like to add Historical Maps to Marble. Ideally one would later on be able to travel through time using a slider later on. For now we concentrate on a more simple approach:
There are lots of very early world maps that are under the public domain. You'll find several on Wikipedia (please add further ones to the list as you find them) and elsewhere on the internet:
-- Magnus has completed this one already
If you find new maps it's always important to check the license. Although ancient maps should usually be under the public domain there are several people who claim copyright on the data due to the image manipulation work they have done as a part of post-processing the scan. No matter whether this is legitimate or not we should respect those claims and look for data that was intended to be published under the public domain (or another free license) and use that.
It's important to note that people like to distribute imagery "free for non-commercial usage". Unfortunately we are not able to use and distribute this data as this would e.g. prohibit most Linux Distributors from distributing our software. So do not use data that is "free for non-commercial use"! It's not suitable for our purposes. If you think that there is a chance that the author might agree to change the license to a less restrictive one then it might be worth to contact him. However pretty often the "non-commercial" wording is provided intentionally this way to allow for a business case of the author.
Maps for Marble need to get provided in plate carrée projection (also referred to as "Equirectangular projection"):
There are morphing tools and HOWTOS on the web that can tell you how to morph ancient maps into the desired projection. It doesn't require too many skills. Just some patience and love to detail.
You can have a look at Spin3D for a source of inspiration. Please consider to use Free Software instead of Morph Man (e.g. xmorph).
The "rectangular image" that is the result after half of the tutorial is what you need.