Not sure if your story is a picture book or a story for a magazine? Author Christine Kohler has an excellent article, "Is Your Story a Picture Book or Short Story for a 'Zine?," regarding this topic on the Institute of Children’s Literature newsletter. She touches on the number of pictures that should be “implied in the text” for a picture book. “Magazine articles require fewer illustrations,” she writes.
In a picture book, she continues, "a story is … told as a sequence of illustrated scenes.” Short stories in magazines, “… do not depend upon the illustrations for clarification.” Read Kohler's entire article to figure out where your story fits.
Elizabeth Bluemle posted a great article, “Standout MG and YA Covers This Month,” plus pictures of book covers on the Publishers Weekly Shelftalker blog. Take a look.
"The longer you put off getting serious about writing, the longer you put off success. Procrastination is a writer's biggest enemy." -- Barbara Seuling