This was requested on another forum and I'm sure that Maura will appreciate.
I hope this helps you to understand how I do that.

My Phal. lueddemanniana, blooming now.

First of all, choose different interesting photos, generally 2 to 4 photos, with a black background, it's easier :

Name:  Compo 1.jpg
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Create a new "virgin" photo, with black background :

Name:  Compo 2.jpg
Views: 699
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Copy and paste (where you want) the 1st photo (in whole or a part) in the new one :

Name:  Compo 3.jpg
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Don't forget to save your work at each step because sometimes Gimp stops working and crashes !!!

For the 2nd one, I decide to cut a part of the photo (drawing myself with the tool) and pasting it in a new layer (useful if the photos overlap at the end). Here, only the new transparent layer is visible :

Name:  Compo 4.jpg
Views: 567
Size:  195.0 KB