The full size one is a tad tricky to read on mobile, but the thumbnail is impossible. I'm on an old iPad, which is pretty big as tablets go, and still all I get is distorted white scratchings. I'm not sure if that's because you resized it, or if you're resizing it in the browser, though.
I think it's very cute, but it does make me think it is for younger readers. Also, I agree, the monsters don't look scary enough for teens. I'd like to see the title bigger. I suppose you couldn't do that without ruining the graphic.
Please take what you need from my critique, and leave the rest:
It's missing the target genre. As everyone else has pointed out, this book says 8-10 year olds.
There are two main issues for this:
1. Your image. It looks like a child, a boy, around 10 or 11, facing off to very cartoonish monsters. The style of illustration is wrong for a YA, and wrong for vampires. There is no indication of Vampires. Two elements need to be included to convey vampires: Teeth and blood. There's teeth, but not vampire teeth. I see them more like monsters coming out of a closet.
2. Your typography. The type needs to be redesigned, as the typeface you've settled on is, in my opinion, overused and not appropriate. If we're talking 14 year olds facing off to vampires you need to be looking at your genre of books to get an idea of how the type looks, like these examples:
If you're going to stick with illustration, you will need to really refine the style and maturity so that you're hitting your target audience, otherwise kids too young might be picking up your book.
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