procedure tform1.button1click(sender: tobject);  
var  
b1, b2: tbitmap;  
c1, c2: pbyte;  
x, y, i,  
different: integer; // counter for different pixels  
begin  
b1 := image1.picture.bitmap;  
b2 := image2.picture.bitmap;  
assert(b1.pixelformat = b2.pixelformat); // they have to be equal  
different := 0;  
for y := 0 to b1.height - 1 do  
begin  
c1 := b1.scanline[y];  
c2 := b2.scanline[y];  
for x := 0 to b1.width - 1 do  
for i := 0 to bytesperpixel - 1 do // 1, to 4, dep. on pixelformat  
begin  
inc(different, integer(c1^ <> c2^));  
inc(c1);  
inc(c2);  
end;  
end;  
end;