Can use PRINT and INPUT instead of PUT and GET if you prefer. Also, you can modify it to decode to an array instead of to the screen if you want to, though it would probably be more efficient to just decode to the screen and GET it to an array.

DEFINT A-Z
SUB SAVEIMG(X1, Y1, X2, Y2, file$)
OPEN file$ FOR BINARY AS #1
c=0:count=0
FOR y = Y1 to Y2
FOR x = X1 to X2
IF POINT(x,y) <> c THEN PUT #1, , count: c = c XOR 1 ELSE count = count + 1
NEXT
NEXT
CLOSE #1
END SUB

SUB LOADIMG(X1, Y1, X2, Y2, file$)
OPEN file$ FOR BINARY AS #1
c=0:count=0
FOR y = Y1 to Y2
FOR x = X1 to X2
IF count=0 THEN GET #1, ,count: c = c XOR 1 ELSE PSET(x,y): count=count-1
NEXT
NEXT
END IF

You could compress the data in an array and even BSAVE it, but then you have to redraw the image some way. My compression requires the same thing.