Lower the opacity of the image.

XD You never understand anything no, just kidding

This isn't going to make sense, but oh well For me, I gather all the Spirit pics I want in my collage and open a new GIMP file. I paste the base (like, the background or something, see my siggie collage in my images for example) and then I make a new layer. I paste another pic onto that and if I only want one part of that pic in there I cut around it with the Cut tool (it looks like a lasso just left of a magic wand :3) and cut it. Then I control the transparency of the image and put it into the new layer. I then erase the excess and make it blend into the background.

Ok, so, hopefully that made sense now...

Thanks! But I don't quite understand 6 X3

1. Go onto GIMP
2. Press the "Open" Button
3. When you have the picture uploaded, go to file again...
4. When you have "file" up, right below it, is an option that says Open in layers
5. Press that option, and upload another picture
6. When you are done, go to layers, and then drag the mouse about half way, or to whatever point you want it to go
7. You can keep on adding layers with that same option, and, there you have your collage! :D

Hope this helps!