[header] How to composite images in gimp [title] Open your images in gimp. [step] If they are not the same size, be sure that the primary image is larger than the one that will be composited. [title] Move what you are going to be adding to the scene, a bear in this instance, up so that it is the top layer. 
[title] Right click on that layer and select add alpha channel. [step] You are doing this because you need this layer to have transparency capabilities.