Ok, here is what I did:
1. Search for a terrain texture [Such as rock or something]
2. Crop it to 512 x 512 [If smaller, you must find a bigger picture]
3. Save it in media > terrains > textures
4. Open a note pad file that says 'defaultTex'
5. Scroll to the bottom and type the name of the picture [MUST be jpg] and save.
6. Go in map maker and test!