Hello dear:) my name is Brittany and I'm a fellow GIMP user:D and I've come with tips if that's alright:)
My first is something that I struggled with when I first started as well, and that's stretched images. What I do it's, instead of trying to resize by hand, I use the scale layer option. When you have a picture and you 'open as layer', you can right click that image and scroll down to 'layer...' And then 'scale layer'. Makes it so much easier:)
Another thing I've seen people mention is matching skin tones. This makes the graphic look more high quality as a whole! And it's a pretty simple fix luckily. What you want to do when you add the photo as a layer (and ALWAYS open as a layer) you can colt just that layer. What I have learned is that it's easier to use gradient layers. Play around with the coloring until all the skin tones match. It takes a little longer, but it looks so much nicer:) and there are tutorials on things like this, just play around:)
As well as the tutorials, you should check out the dueling arena. It gives you a chance to make graphics without worrying if the requester will like it. And it's just fun and challenges you to try new things:)
If you have any questions, feel free to pm me (although as an MA, I can't promise to always know the answer
) and keep graphicing!
P.S. Sorry if I was harsh. I just know I love critique, and it helps me get better
P.S.S. Sorry about the typos, I'm on my phone