Aquí explican como comparar dos imágenes:
Mediante Scanline
Código Delphi
[-]
function IsSameBitmapUsingScanLine(Bitmap1, Bitmap2: TBitmap): Boolean;
var
i : Integer;
ScanBytes : Integer;
begin
Result:= (Bitmap1<>nil) and (Bitmap2<>nil);
if not Result then exit;
Result:=(bitmap1.Width=bitmap2.Width) and (bitmap1.Height=bitmap2.Height) and (bitmap1.PixelFormat=bitmap2.PixelFormat) ;
if not Result then exit;
ScanBytes := Abs(Integer(Bitmap1.Scanline[1]) - Integer(Bitmap1.Scanline[0]));
for i:=0 to Bitmap1.Height-1 do
Begin
Result:=CompareMem(Bitmap1.ScanLine[i],Bitmap2.ScanLine[i],ScanBytes);
if not Result then exit;
End;
end;