To do this, first save the picture to disk (using BSAVE 'filename',A1024,L1024), then (under program control, and after the LoRes screen has been turned on with the GR command) load the picture using BLOAD 'filename'.
With ProDOS, however, the area allocated for LoRes screen memory is protected, so the above procedure used for DOS will not work with ProDOS. To get around this, undo the protection byte, load the screen the reset the protection byte back to the original value. Below is a short program that will allow the ProDOS user to BLOAD a previously saved LoRes screen.
10 A= PEEK (48984)
20 POKE 48984,0
30 GR
40 PRINT CHR$(4);"BLOAD FILENAME"
50 POKE 48984,A
60 END