As far as I know DoubleBuffer is only for Windows, on mac the OS does it for you. So turn that off if you're only doing mac.
Your pictures are getting crisp because it's being drawn over many times, each time the anti-aliased edges get darker and darker, filling those pixels in to solid black.
I'd need to see some code to know what's going on. My guess is that you're drawing to the Canvas' Graphics from
outside the Paint event as that's the only way I know to not erase the previous drawing. While this still works you should refactor your code to draw only in Paint, this would fix your problem as the Canvas is cleared each time Paint. For a quick and hacky fix without refactoring just clear the canvas before drawing...
MyCanvas.Graphics.ClearRect(0, 0, MyCanvas.Width, MyCanvas.Height)
MyCanvas.Graphics.DrawPicture(myPic, 0, 0)
Really though move you code into Paint. It might seem weird at first but it's actually cleaner once you get the habit.