The banner looks good to showcase the the cast; at a glance, I get the feeling it's a light series about friendly demons? Dude on the right is a little creepy tho, just kinda standing there all alone... Dull color palette looks good on the characters (creature designs are great btw) but the sky and... white parts? could use more work - maybe a darker gradient, pattern, texture, etc. You can see from this thumbnail that the blues all blend in together.
You might want to move the title up, it's getting cut off on widescreen:
Probably gets cut off from the side on phones so a centered placement would be a simple fix. Maybe add a little tagline if it suits.
Icons are just a small image with a square crop so, simple content and composition are enough to make it look good at various sizes. If you want it to look like a phone app or something, beveled edges, transparency to round corners, thin frame, fake shine, etc. are some effects to play with. You can bump up the color saturation if you like but overdo it and it can easily look childish and annoying.
Hope this helps.
