High contrast, filling the entire space
Black and white with good contrast is fine to use
Logos are OK-ish, not the best as they don't fill the entire space
Symmetrical targets don't work well
The entire space, or at least each corner needs to be used
Wishy washy low contrast or blurry images are not great
The most important image, the stronger your target image the better your onvert will stick. Strong constrast gives the best results filling the entire space. The ideal dimensions are square 512x512pixels.