Gandalf said:
Your upload had significant compression artefacts, specifically the blocky kind that happens with lossy video compression (you can see it most clearly in the top left). Meanwhile the image you're showing as comparison shows the paper's texture. Watercolor paintings usually use rough paper which adds a natural grain to the image.
First thank you for at least deleting something (that wasn't uploaded by me).
From what I've understood the "low quality" is concisely defined and extremely obvious.
Either way, I am suggesting that you automate a "low quality" check when uploading, like you already have duplicate checking.
Off course, it won't catch 100% of the low quality uploads, but it will prevent blind people (like myself) from uploading completely.
Or if you can't do it, make a new forum thread, or web-page, post rules, post examples, side-by-side comparisons, or simply put a more transparent wallhaven. If we catch an official in a sin, we want his uploads taken down. Thanks