50% opacity probably works my manually interlacing two images, then reducing the size to 50%. You can also do fun stuff like gradients in MS Paint: start out with a square canvas, divide it diagonally and fill each resulting triangle with a different color. Then use the "Stretch/Skew" dialog to horizontally (or vertically) stretch it all to the minimum (5%, if memory serves me right).

Edit: Actually, I suppose the minimum stretch was 1%. So the square should be 100x100 in size :)