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You can add two or more images together in GIMP using layers. Using layers opens a world of creativity for the artistically minded.
Layers are separate images stacked on top of each other to make one picture. Usually there is a background image with one or more images placed on separate layers on top of this background layer. Placing images on different layers allows you to work on each layer separately.
In the pre-digital imaging days, it was possible to add images together by cutting and pasting. When you wanted to make up an image from parts of other images, you cut out the images and then pasted the images on a sheet of paper. Using this cut-and-paste method, you could control where each part was placed, but you couldn’t change the size of any part or its transparency. If you made a mistake, you had to find more copies of the images and reassemble them.