I took a class on this. It's fun and easy. After you've done your picture on muslin or cottons, whatever you want to use as your base. Lay paper towels over the crayon picture, iron with hot enough setting to melt the crayon into the paper towel, replace towel until you get all the wax off. (No more color comes up on the paper towels)
Don't try to scrimp on the paper towels, you'll smear the picture if you run the iron in normal ironing motions. I set my iron down, leave for a minute or less, then pick it up and set it down on the next section.
You'll love this easy method, but if you want bright bright colors you may want to color and iron more than once.
Happy 'fast' picture quilting!
Sharon