(GET-IMAGE upper-left-point lower-right-point)

Returns an image-string containing all bitmap data for the rectangle enclosed between the two given points. Don't try to look at too big strings of this type, or you might have to wait for a long time until you reach the end, especially if there is a bell every two chars...

You should use IMAGE-SIZE to figure out how much memory a bitmap needs, and never allocate an image of 32Kb or more.