One thing readers hardly ever see are a traditionally-published author's ideas about what sort of cover art should go on their books. If the author is permitted to have any input (which very often they do not) they communicate behind the scenes with their editor or cover designer. I've been fortunate in that most publishers have at least invited me to contribute my ideas, and in one case even borrowed my own cover art design for an e-book release (and this sort of thing is extremely rare.)
Over the years I found what worked best with editors is to send sample images along with the cover art ideas. Here are two art idea outlines that I put together for my editor at Pocket to show him what I thought would look great on my books:
Harry's Charm cover art ideas
Clockwork Wolf cover ideas
Unfortunately none of my ideas were used for the final covers, but what I like and what is marketable are often two very different things, as you can see here with my cover concept for Dreamveil, and what actually ended up on the the cover:
Still, it was fun to put them together, and it was nice of both publishers to invite me to contribute my ideas.
If you're ever asked for cover art input, here are some tips:
Consider your brand -- if you want to set yourself apart from the herd, go for ideas that present an original look.
Make up a prototype -- to add visual impact to your presentation, make up a mock book cover like the one I did for Dreamveil that incorporates your idea.
Offer more than one idea -- it demonstrates you're flexible, and fixating on a single cover concept almost guarantees disappointment.
Refer your favorite artist -- I often recommended artists I love to publishers; if you have someone in mind provide contact information or a link to their web site.
Think about color -- when I was publishing the original Darkyn series my ideas for color themes made it onto two of the books, Night Lost and Evermore.
One final tip -- if an editor ever shows you example covers and asks for your feedback, no matter how lame they are, try to be polite. I was once very candid about how much I disliked all the cover art examples an editor sent me, and I didn't bother to mince my words -- at which point she got very miffed and informed me that she had picked them out because they were all her favorite types of covers.