Hi twa96 and

to Gimp Chat.
In addition to ofnuts' response, you would normally place your background image on one layer and your foreground image on another layer. This will allow you to resize the foreground image independently of your background image. Saving the file with a .xcf extension maintains the individual layers. Exporting the image as a .png or .jpg will combine the separate layers.
As already mentioned, one caveat is that when you resize an image up, it becomes more pixelated. It's better to start with a larger image and resize downward, rather than trying to resize a smaller image upward.