Enchanted Learning Intended for KG to Grade 3 learners, some of the images are too "babyish" for older students. Images are mainly black-and-white. You must be a member to view the full screen printouts ($20 for a single user; $125 for a school subscription for one year).
101 Picture Prompts to Spark Super Writing! by Karen Kellaher is a paperback with photos, artwork and cartoons. Includes suggestions of how they can be used to prompt story-writing in Grades 3 - 5.
Story It has some picture prompts for Grades 3 - 6. Some have a beginning already written out for continuation.
ETTC This site provides some action photographs - fire, sports, etc. - suitable for middle or high school.
Bright Ideas provides a photograph with a written prompt. Suitable for middle school.