Tomie dePaola’s Pancakes for Breakfast has illustrations but no words. With illustrations as props, students will not have to figure out what to write about but rather how they want to tell the story that is already there. You’ll find step-by-step instructions for using the writing process with this project here.
The alphabet can be a catalyst for imaginative writing. Find an idea for using the alphabet here.
Book jackets can ignite all kinds of ideas. The book jacket becomes the cover for your student’s story, which he can later compare with the author’s original. See more here.
A series of pictures can give your child the nudge she needs. Here is my favorite set.
Roald Dahl does a great job of showing us Mr. Twit with his words. Your students can, too. See how here.
Maybe entering a contest will trigger some inspiration. I've listed some possibilities here.