Hello ocratato and welcome.

I will start in the end of your post if that is OK with you....

(The picture with the link https://ibb.co/dtXJ2x doesn't quite look right - I think the perspective of the boat doesn't match the rest of the scene.)
About that wallpaper....That is mix of photo that was taken from the boat on the water close to the Tel-Aviv (Israel ) shore 2 weeks ago,
And the ship is German from the 40's-50's.

so maybe that did not pass your look because of the ship and the new world On the horizon,
But the ship cut the water nice,and I had to spin the ship in A way that it will not cut the background.

Hope that Answer your Question about that wallpaper.

Would you care to give a brief description of your technique ?
If you will go back to the beginning of the Gimp Package For ALL,you will see that I made some quick Guide for everyone.
technique - everyone develop his own way to work with Gimp.
when you say Open Source you say it all,every person chose the way he prepare the wallpaper.
one can start from the background,and Other can start from layer Options...and so on.

this is all about the way you chose to do your work,and what you get in the end of that way of working.
But I will tell you that,First of all will be allot of working with the software and learn the Options that you have in hand.
Then comes the part of experiencing the development part with images,plug in's, Other Options and so on.
what you need to pay Attention to carefully will be the balance of the colors in your work.
As that can be issue with the final image and negative results OR conflict that will get your eye in the bad way.

Here is A Tip from me to you and to all.

Do not be afraid to try things and explore the options to work with the software !!!
as if you made some test and it did not work for you, then you can go back with few clicks and start over.
As one that working with Gimp software for 12 years or more,I can say that:

1- learn the place of each one of you effects/plug-in's/Layers position in your work...
if you ask me start with the first step that will be the size of the wallpaper+background "Black".
why Black? because that will Highlights your work and let you see more clear the page and issues on the wallpaper.

2 - if you have image,play with it,stretch it and manipulate it with your software options.
then see if that not makes any issue with other elements/effects in that area.

3 - Real Tip from me will be in the part of the Layers.
on the right side of the software you have the layers that you have added to your work,
Now that place is not " Holly place ",there you can change the place of your layers with just drop down layer to Other place.
now that will change things on the page (wallpaper) that you working on.

Hope I have Answer your Question ocratato .
As for real Answer and Guide you and Others I will have to write A book on Gimp.

thank you for the question.
God Bless.