I'd paste each of your photo extracts as a new layer:
Edit -> Paste as -> New layer.
Or if you want to import each photo from a whole file each on a separate layer:
File -> Open as layers.
To manage the layers, bring up the layers dialog (if not already displayed):
Windows -> Dockable Dialogues -> Layers
You can move each layer about independently with the move tool.
To export your work make sure all the layers you want are visible (eye icon in layers dialog) and then export the file in the format you need e.g. png, jpg etc:
File -> Export as.
Also save as an .xcf file so you can go back and adjust in future as the individual layers will still be there in an .xcf file:
File -> Save
Personally I wouldn't merge the layers as that reduces your ability to go back and edit. If you want to merge them you can right click on the layer in the layers dialog and choose Merge Down or Merge visible layers.
Alternatively if you want to paste into a layer then click on the layer in the layers dialogue and then when you paste the results go in that layer. Look out for the dashed line round the layer that's the layer boundary.
To make a new layer click on the Create a new layer icon at the bottom of the layers dialogue, or use menu:
Layer -> New layer.
Here's a quick example showing the layers dialogue on the right. There are four layers, background, text and two pasted layers with photo extracts. The active layer is Pasted layer #1.