I would'nt go with just hamburgers, maybe something like healthier ways to cook the fast food you love, with ingredients that cut calories from the recipe, that would go well with people trying to cut back. You could use pictures that show how to cook the recipes, like you want to. I think the picture thing is a good idea! Most of the cookbooks that my parents have only have pictures that show only the finshed product, not how to get there, you may also want to include some videos of the hard to explain parts, that would go over very well. I hope I helped some and good luck!

PS:I would do what these other people recommened, looking at the competion instead of just diving right in once you have a narrowed down topic.