I do think it is neat as an artwork and it has brilliant lighting-shading with all the colors It conveys the fantasy element well and we can already see the main characters The characters and their pose are clear and can be distinguished even in mobile, it's a good job!
I think it can be better if you use less empty spaces, and focusing the zoom to the character. Maybe it needs an alteration in composition such as Shade is moved slightly closer to flare. I think it can be a bit hard due to their height difference to preserve the exact pose and angle while being zoomed in, so this is just a suggestion and take it with grain of salt.
Is the "BS" thingy supposed to be the logo? (Burning Shadows isn't it) I think if you did intend to make it a logo, you'd like to make it bigger and more prominent. Also it is me being nitpicky again, but is it your style to make it handwritten (because it seems so in the banner)? While it is certainly okay to as mine too, I think it will look better if it's tidied and polished a bit so it won't look like you scribbled it in a rush. I actually prefer the style of writing you used in your thumbnail, it has more dimension and magical ring to it.
This is just an opinion of an unqualified so I'm most likely be wrong.