Bmp2Scm is a pretty interesting program; its most famous use is in the "Sexy Pics" series. Perverts
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Basically, you input a picture, then take a blank 128x128 (Has to be this size) map, detect the color settings of the image (The color of each individual pixel that makes the picture) and select certian tiles to match up with the pixels of the picture.
As to how to select the right tiles (Each tile has a number), refer to the little help guide in Bmp2Scm.
A good idea to keep in mind is to have an image editor on hand (Adobe Photoshope is good) to reduce the amount of color a picture uses so you wont have to replace 1000 colors with different tiles.
EdiT: Found a post with more data on this program:
Bmp2Scm