DRAWING

Maybe a using PEN mode can give the effect you need.

Try a variation of this simple code:

WINDOW 1
'--- the boxes would represent your pict
LONG COLOR 65000,0,0
BOX FILL 10,10 TO 200,100
LONG COLOR 0,65000,0
BOX FILL 10,100 TO 200,200
LONG COLOR 0,0,0
'--- you could highlight a frame around your pict
'--- when the cursor is over the pict
PEN 2,2,1,10,0 'using mode=10 should replace the original pixels
BOX 15,15 TO 195,195
DO
  DELAY 200
  BOX 15,15 TO 195,195
UNTIL FN BUTTON

Here's a minor modification of the previous to show the effect with the mouse position.

WINDOW 1
'--------------------------
LONG COLOR 65000,0,0
BOX FILL 10,10 TO 200,100
LONG COLOR 0,65000,0
BOX FILL 10,100 TO 200,200
LONG COLOR 0,0,0
PEN 2,2,1,10,0
BOX 15,15 TO 195,195
DO
'this could be in your dialog handler with cursor events
  LONG IF MOUSE(_horz)>10 AND MOUSE(_horz)<200 AND MOUSE(_vert)>10 AND MOUSE(_vert)<200
    IF new%=1 THEN BOX 15,15 TO 195,195:new%=0
  XELSE
    IF new%=0 THEN BOX 15,15 TO 195,195:new%=1
  END IF
UNTIL FN BUTTON