GIMP: how to create a collage where the photos can be repositioned within frames?

Question

I'm trying to create a collage with 3 photos in GIMP, and I want to be able to reposition the images within each frame, not just crop them to fit the frames. Here is a sketch of what I want it to look like:

In short, I want to be able to move each photo independently within each frame (black box) without them showing on the other frames. Here's what I've tried so far (and what problems I ran into):

1. Create background, add layer mask from selection, import photo as a new layer below (possible to move photo inside frame, but multiple photos/layer groups block each other when added with the same background - only one layer group shown at a time)
   
2. Create transparent layer, fill selection with foreground color to create a frame, import photo as a new layer above, add layer mask from selection (allows multiple photos to be shown on the canvas, but the layer mask inhibits repositioning by cropping the photo to selection)
   

Any help would be greatly appreciated! I'm using GIMP 2.8.10 on Windows.

Answer 1

Try this:

Import each photo in a layer, create a layer mask, and draw your frame as a white rectangle on the mask. To position the photo, select the layer (not the mask), pick the Move tool, select all (ctrl + A) and float the selection (ctrl + shift + l). This way the mask will stay in place as you reposition the layer contents. (Anchor the floating selection after you are done - ctrl + H)

If you have to do this several times, it may be a simple thing to script the steps of create mask, draw frame, float selection for each layer - so you just have to position the photos - or even have a template with the frames, and the script would open each photo in a suitable layer, preserving the mask.

If you want to do that, please, check my GIMP answers on stackoverlow.com (as these are related to code).