Hi there, In the past people used SCRACC (A SimCity terrain and region creator), but in recent years that has been hard to find online. One program that I found that works wonders is HME (Height Map Editor) found
here and a good example is this
map (created by me using HME).
If your interested, I made a small chart that denotes the locomotion height values in HME and attach it later. I found it useful when making my maps the past.
EDIT: I must misread the question, your asking how to put the bmp in to mapgen. Simply run mapgen, it will ask you for a bmp file, then direct it to a folder with the bmp, follow the prompts and hey presto, your landscape is done.