I made this Monarch Butterfly Collage with 5 photos. The first, and background layer, was a photo of painted daisies. I gave it a textured filter.
The remaining four photos were close-ups of a single Monarch Butterfly. I put those photos into 4 layers, gave each of them a border, and positioned them on top of the background photo.
Layers, in GIMP or any other photo editing program, are like a stack of transparencies. There are many ways to alter each layer, without affecting any of the other layers!