now, if the canvas (the white background) is not big enough, take your mouse pointer to the bottom right corner to where you see. alternatively, you can just right click on the image that you want and select open with >paint. select one of the images that you want to combine two pictures. in the paint window, click on the file >open. In this tutorial you will see how to open more than one image in paint software, you can add multiple images pictures by using this software.
select the image you want to set as the base (background), then click open. now you would see the image has imported to paint. Select the image you want to insert on top of another image.