I realize this is slightly OT for this forum, but I've got nowhere else to go....

What is the name or what is it called when you take a bunch of photos together , obviously computer done, sorts out the colors and makes ONE big picture?

You know what i'm talking about....from a distance its a picture of whatever, a famous person, but when you get up close you see that picture of the famous person is made up of hundreds or thousands of individual photos of different things.

My questions:
What is it called?
Where could I get it done?

THANKS!