Patrick, I think that is just a rule that you made up. I had read a lot of books that has blurbs that doesn’t reinforce the message of the cover. Gosh. There are even books that has a plain cover. No blurbs. No picture or anything, just plain paper or cloth, sometimes with the title printed on them.
Designing an illustration is very difficult. I have been making digital illustrations (2D and 3D) as a hobby and they are very difficult and time consuming.