AKbmp2map = your best friend right now.
It doesn't do a bunch of things I wish it did but its still a very useful little program.
Bear in mind as Batz said white=high black=low. Taking this one step further I set my max alt at 12,800ft. This means every level of the greyscale is 50ft and you can draw the entire map knowing the altitude of every single point, (ex. RGB 19,19,19 would be 1000ft).
Gatso