First comment! Woot!
Just wanted to drop by and leave your gallery a bit of love. I'm so glad you've decided to join the dark side and come do graphics with us! I promise you're going to have so much fun with it!
Wow! I'm honestly super impressed with what you have to start. You are already a million miles ahead of where I was when I first made my gallery. I'm particularly impressed with how well you create backgrounds already. Both your stock and texture use is crazy good for someone just starting out. That's so great too because it's such a hard skill to learn if you don't get it. I struggle so hard with stock, so to see your top banner with different pieces blended together really blows my mind.
I think you're doing a great job of picking HQ images as well, which is not something that everybody is good at right away. As long as you keep the rules in mind about only making images smaller, not larger, you shouldn't have much problem there.
I'm also really impressed that you've already experimented with different sized graphics, different techniques, and different bases. So many artists, particularly new ones, tend to only use texture or only stock or only do certain sized graphics, so it's really great that you're not limiting yourself. Experimentation and trying new things is honestly the best way to learn and you'll pick up so many cool little tricks doing that. To help push yourself to keep trying new things, I'd highly encourage you to check out the Dueling Arena which has all of the different challenges that are posted through the month. That's a great way to continue developing your skills.
Now, I know you're still learning a lot of different techniques, so I'm not going to be super critical in my crit part. Instead, I'm just going to give you a few things to work on based on what you have here in your gallery.
1. Hardcuts. Be careful about your hardcuts on images. On this banner here, if you look at Dan, you can see a thin line of white almost all the way around him, which is leftover from the background. To clean something like that up, you can zoom your view in to about 400% and then take a hard eraser (sized 30 or so) and erase away the remainder of the edges. Also, on this banner here, on the larger image of Hermione, you can see that part of her hair has been either erased away or was cut off in the original image. You can either go in with a hard eraser and shape it so it looks more natural or hide it behind the edge of the banner or another image. The flowers on that banner also have some of the remainder of the background as you can see some of the grey around the edges of the roses.
2. Contrast. You're doing a great job of using photos where the skin tones match well, so that's a really good step, but one thing you are missing is contrast in your images. If you look at the people in your images, most of them have a sort of foggy, light look to them. That's because the brightness/contrast is too low. You can adjust this by going to the different tabs at the top of GIMP, selecting colors, and then selecting Brightness/Contrast. Experiment with the different levels a bit, but generally if you take the brightness up, you want to take the contrast up some as well. In particular, your images of Lily Collins need the most contrast where as the images of Emma Watson on the second banner could do with more brightness.
3. Text. Okay...so text is a nightmare for everyone. Even super experienced artists still struggle with it, so don't feel bad at all. The best thing I can suggest is to download as many fonts as you can. I mean like 100s of them, so that you have tons of selection and things to experiment...and then just play around with what looks good on each graphic. Keep in mind that softer images might be better with scripty, cursive fonts and harsher, edgier images might go better with distressed, bold, or blocky texts. Also, as a beginner the best strategy for text color is to use colors that are already in the graphic. On your top banner for example, the red on Bury Your clashes with the rest of the graphic and draws attention away from the focal point. Maybe picking one of the green colors in the background would've fit a bit better. Also, Bury Your should be a bit smaller and closer to the word Memories.
4. Focal Point. I know we've sort of already talked about this a bit with the graphic you're working on, but just as a sort of refresher, try to remember to vary the sizes of the people in your graphic. If you look at most of what you have here, everyone is pretty much the same size, which can cause the eyes to shift back and forth instead of focusing in on a central image. Does that make sense? So like on your second banner, I see that one Hermione is smaller than the other, but they're still on the same level in terms of height on the banner. You could make small Hermione a bit smaller and move her down so a bit of the background shows above her head...creating different levels in their height.
Honestly though, you're doing so great! Don't feel discouraged by crit because everyone that gives it is only trying to help you improve. And also, don't be shy about reaching out to people here and asking for help. There are so many higher ranked artists who are willing to teach you cool techniques or give crit on a specific graphic, so if you introduce yourself and start getting to know people, you'll find a wealth of resources here.
And also, I'm always here to help if you ever need it (although I'm still a pretty beginner artist).