I don’t know anything about drawing backgrounds or landscapes but these came out ok
P.S. You should try Krita instead of GIMP, I switched a while back and it's absolutely awesome: http://krita.org
I use Krita sometimes but I tend to go back and foth between programs.
Mostly because I just can't seem to settle on one.
In Krita, make sure you make full use of the right-click palette and shift-scaling. I keep two brush presets in that palette as dedicated erasers, a couple of painting brushes, one for sketching and one for inking.
I use to use GIMP for all of my work, but since about a year ago Krita has become vastly superior for painting in every regard and it's been rapidly improving with every release.
Link
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Good to see you practising! These came out really nicely, although I do has some thoughts about the trees.
Most trees don't grow straight and symmetrical, so using an S curve for the trunk and making the canopy more irregular will usually makes it look more organic. If you grab some reference of some trees, a good way to study them is to draw out the overall shape of the tree first, like instead of drawing the trunk, branches and leaves etc. you just pretend the whole thing is wrapped in paper and only draw in the outline of that overall shape first. Once you have the trees personality, then it's easier to draw the details over that.