If you still have the original image, I might suggest inking it, and then cleaning up the digital copy so it looks neat. Currently it looks like the tea color might be intentional, or might just be bad lighting. The latter is likely to turn people off, as they'll expect lower quality. Either way, bolder lines and higher contrast will probably show up better anyway.
Alternatively, if you wanted to draw something new, maybe think about what kind of image could better reflect your comic? The current thumbnail says "this is a story about a cute girl" which is not bad, but pretty basic. It's good to have a simple thumbnail, but it doesn't hurt to think "How can I make this more interesting??" Could her expression suggest a more interesting emotion? Could her body language?
