Anyway, I got three shots of the daughter of our friends as she was jumping into one of the pools and I wanted to combine them into one shot.
Here are the three shots:
I decided to use the first image as my base image - only pieces of the other two would show up. I started by loaded the three images as layers in GIMP. This essentially means that they are stacked, one on top of the other, like cards. However, only the image on top shows.
The next step is to define what you want from the layer below to show through to the top. This is done by creating a layer mask. What this does is essentially makes a hole in the top layer for the layer below to show through.
I started on the bottom two layers - the mid-air and the splash. I added a layer mask and then used the paintbruch to outline my splash - this is the part I wanted to show through to the layer above. Once that was done, I added a gaussian blur to the layer mask - about 10 pixels. This makes sure I don't get any hard lines around my cut-out and it blends a bit better. Once I had the splash showing through, I used the airbrush tool to touch up parts and blend the edges a bit better. The airbrush tool works well because it is a soft effect tool - you can slowly blend things together, unlike the pencil or paintbrush, where you will get hard lines.
Having done this on the bottom two layers, I merged them, copied the layer mask into a mask for the top layer, and then just added the mid-air cut-out to my layer mask. Blend, touch up, then merge layers. Voila! You're done!
The only other thing I did was slightly shift the bottom two layers, otherwise the three stages - standing, mid-air, and splash, would all be pretty close to stacked on top of one another.
A good way to guide your mask-making and layer movement is to adjust the opacity of the layer above - slide it down so you can see the layer below.
And here is the final, with all three combined:
