Today, I will describe a nice feature that you may find in GIMP 2.8: combined image opening.
In the "File" Menu, you can find a new command, "Open as Layers..." (Shortcut Ctrl + Alt + O), right below the old "Open..." one.
In this case, a dialog window will open, that allows you to select several images at a time, no matter sizes or file formats. As usual, you need to keep "Ctrl" pressed (or rather, Caps key) to mark several images, then hit on the "Open" button.
GIMP will open all images in a single file, each image in a different layer. This may spare you some work when doing some collage or composition. Initial image size will be that of the FIRST image selected, but you may see in a dotted line the size of the other ones around the canvas, if bigger. You may expand the canvas (from the layers window) to the biggest image, if needed.