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 :
Create a new "virgin" photo, with black background :
Copy and paste (where you want) the 1st photo (in whole or a part) in the new one :
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 :