The next big thing you need to know is Layer. Simply they are pictures placed on top of each other. This makes it easy to combine images and control one part of the image without affecting the rest of the image. So simply the layers on top will over lap the layers below. If you want to move a layer its as easy as dragging it above the layer you want it to over lap. Now all base images will have a background which will over lap every thing within the area, if on the image you don't want that to happen you will need to use an alpha channel, this will let you remove the part of the image that will be transparent on the other layers so the layers below will not be over lapped.