The overlayed image will render at the highest provided resolution compared between both images. For best results try to keep the resolutions of both images relatively the same. It is also recommended for both images to have the same aspect ratio.
The first parameter, or image that you input will be taken as the main (Or base) image. The second parameter or image that you input will be the one that is overlayed. If you have a transparent image, we suggest adding it as the second image.