To be completely honest, a lot of it is getting the feel for it. Art is difficult to teach because there isn't a specific formula to follow, it deals a lot with the artist's imagination and personal abilities. My advice, just play around with stuff. Use blur tools, opacity, several layers, etc. Another tip, specifically for digital art, is when ordering layers, put the foremost things (ie anything closer to the viewer, something with nothing in front of it) at the top and background things (stuff with things in front of them) at the bottom. Also, its always helpful to have lines and color in different layers (with lines higher up on the list) because nothing sucks more than messing up a line while adding color (and its really hard to avoid empty space and not hit a line; your color layer will almost always go under the line). And also take advantage of locking and hiding layers! It'll save you a lot of time!
Can you explain what these thumbnails are for? Though not knowing their ogal, my first comment would be that they're too busy. The could make nice wallpapers, but as such small scales, they're very busy and noisy, a small thumbnail needs to communicate simply and effectively and you just have a LOT going on in most of those.
I really am unfamiliar with GIMP simply because I've tried it a few times and found some of the abilities of it sort of eh. Like, part of the problem is GIMP is awful at resizing things.. making things look smaller even always comes out looking like a mess. So one tip I could give is to do any cropping/resizing on a different program or even website. Tons of image hosting sites allow you to edit an image afterwards, and have often found better results with their cropping at least in comparison to GIMP.
I wish I could help you more, but I'm so unfamiliar with that specific program. There are certainly some tips I could try to help pass with my meager skills if you get your hand on a Photoshop. Sorry, I'm sure that doesn't help much.