I recommend not working on a white background unless your entire game takes place in a white avoid. Pick a value that's closer to what is likely to be in the game. Ditto for presenting your stuff online. PJ has a midtone grey background that works well for most things, so perhaps you should post your images with a transparent background.

White makes it hard to balance your values and to know when you have too much or too little contrast. It makes it hard to give feedback too, for the same reason.