@Listeczek
My impressions: I like that the background doesn't distract from the character, and, thanks to your color choices and shading, her features are pretty clear even when the thumbnail is shrunken down. I definitely get a retro comic vixen vibe. The palette makes me think of Starfire & Poison Ivy. Looking at your first episode, I see that she's a siren, but I wouldn't have gotten that from the thumbnail. Something about the proportions of her face seems wonky — almost like her head and her facial features are at slightly different angles — but I might be wrong about that. At first glance, I'd assume she's the main character, some sort of seductive alien anti-hero. If that's what you're going for, then great!
Suggestions for improvement: I don't think it necessarily needs improvement. That said, you could possibly add some visual theme to hint more towards the plot or even just the genre. From this, I can't tell if it's a sci-fi adventure, fantasy romance, supernatural thriller, or anything in between. I'm not sure what the significance is of the floral motif in the background, so it feels almost like wasted space. Obviously you don't want to steal too much attention from the character, but maybe that would be a good place to add a little more info.
Disclaimer: I'm a novelist, not an artist, so take my opinion & advice with a grain of salt.
@aqua03 I don't mean to skip you, but I believe the rules are you're supposed to give feedback to the person above you before you ask for anything. I just wanted to make sure @Listeczek didn't get missed.
Thumbnail for my novella:
I made it myself, but I wouldn't consider art my strong suit. I'd appreciate any feedback and/or advice!
I converted the thumbnail from this cover (also made by me — click for full size):