I rarely use GIMP and whenever I do I need to re-learn basic functions. I recently wanted to place two images side by side. I opened one image. To make space for an adjoining image, I used Image>Canvas Size to increase the canvas width. To add a second image I used File>Open as Layers. I could then scale the second image to match the first using Layer>Scale Layer. To open the tools menu, use Windows>New Toolbox. To open the Layers menu, use Windows>Dockable Dialogs>Layers. Right click on a layer to add and Alpha Channel. Once the two images are combined, use File>Save As.