Idea form ElPsyConGree. Using alpha in image to hide one image from another. One image can be seen in white background, and another can be seen in black background.
For now, those two image can not be to bright, and only greyscale can be supported. So you had better convert the image by yourself to check the visiblity.
For now there is no UI, so you can only put two image in the test folder and rename them to test-1.png and test-2.png. Of cause if the image format is jpg, you have to change the code: change the path in test function to your imgae.