Hello!
My name is Daniel Shubin, I'm a Russian graphic designer now based somewhere in Eastern Europe. My main areas of interest are character design, magazine graphics, posters and editorial illustrations.
I always open for cooperation.

I was asked once in an interview:
"How do I know that this illustration is of high quality? Some like it but others don't.
There's got to be something objective, proving the high quality of the image."

Let's just put aside for a moment that the illustration is a work of art, an individual product of the artist's work, and a matter of artist's taste. It is theoretically possible to use any created image as a cover or a poster. It is hard to distinguish the criteria of good illustration. There is no perfect recipe. However, we can say positively that the image should satisfy objectives effectively. For example, if this is a poster, then the idea the illustrator is trying to make here depends on his work with the format. A poster is taken in from a distance, like any other outdoor advertising. Thus, fine details can interfere with the reception of the idea. On the other hand, if the size of the page allows, why not fill the image with details? It will be more interesting to look upon the page. This is the way the artist helps the reader perceive the text of the article more efficiently and remember the information received. The illustration helps reach effectively the client from the customer through a cover of a book or a funny character on a package of cookies.
We all live in an era of visual oversaturation. Every minute Internet users exchange billions of images. However, only a few become legendary memes because their creators were able to get in the mood and find an artistic solution that echoed in our hearts and made us share this picture with friends. That is why, efficiency (success) of an illustration, how well it helps meet the goals (push up sales, scare in pages of books or laugh on the Internet) are objectively very important criteria for its high quality. All that relates to the visual part and technique is highly debatable.