First up, I took a couple of night shots and combined them using a layer mask to let the moon shine through. Both shots were taken the same night, but I couldn't get crater detail and the cloud haze in the same shot. Not a very smooth transition, but since I was aiming for something a little unreal it works OK.
Next, I took a daytime shot (from a completely different location) and again used a filter mask to clear out room for the night sky (much more fiddly than just a circle selection this time!). I applied various filters to make it look a bit more night-y. I also cloned Lily out of the shot.
I started with a very rough sketch to ensure there was some depth in the shot. That boxy camera stuck around most of the way through, but I found it an unneeded distraction.
Miriam and Alice got a makeover and moved much closer the foreground to emphasise the depth. I used a combo of different reference shots (mostly basketballers) to get suitable action poses:
Finally, I moved the moon further out from the trees (as easy as dragging that layer a bit more to the right - I love layers!), drew my version of Liv's 3D monster, with a textured brush for fur-like shading, and voila!
All up this took a few hours, since I had to relearn the GIMP functionality for layer masks, try out different filters and do the drawing itself, including painting in white behind the sketch to make it visible, since I always draw on a transparent layer. Absorbing, tiring, and I'm very pleased with the result :)