A Picture stores the state of the Graph Screen in a pixel-by-pixel manner. It does not store any data about the screen other than which pixels are on and off. This is the opposite of the GDB variables, which stores data used to recreate the picture.
Pictures are often more useful than GDBs since Pictures can store effects of Pxl-On( and other Drawing Commands. Additionally, after Pictures are recalled, they do not have to be drawn like GDBs do. The downside is that Picture variables are larger than GDB variables.