Open GIMP and open the first image. Open another image either with Preview or GIMP, select and copy the image, then paste into GIMP. In the Layers menu the pasted image will appear as a Pasted Layer. Select that layer and click on Create a new layer in the lower left of the window (looks like a blank page with a plus sign in the upper left). The pasted layer should now be selected. Move the layer to the new position. It won’t appear beyond the limit of the old image unless it’s created as a new layer. Then under the Image menu select “Fit canvas to layers” and the second image should now appear in a new, larger canvas. Repeat for additional images.