Here is an article which I wrote for Hubpages that gives a full answer to your question:
What Images are in Demand by Stock Agencies?
Preferences of every stock agency are different due to the aprroach to customer market and destribution. There are also other factors involved in alteration of interest, like seasonal changes and state of events in the world. However there are a number of concepts and ways of image presentations that are in demand almost everywhere.
To be successful in stock you need to create images with clear concept which will communicate a message to the viewer. Stock photos are meant to promote business, therefore it should be clear what every photo represents.
Currently highly requested images at stock agencies are on subjects of:
• Active people: biking, hiking, doing sports, exercising.
• Business and finances: photos of business people, money, banking, e-commerce.
• Emotions: depression, worry, stress, disappointment, loneliness, pain, content, happiness, joy.
• Food: well executed with bright colours photos of prepared food as well as food ingredients.
• Health and medicine: anything related to medicine and health, objects and photos of medical staff.
• Relationship: photos of happy as well as sad couples, photos showing different generations of people, parents and children, interaction between people.
• Technology: computers, e-books, tabs.
• Travel: images of hard to reach places.
• Workers: images of people working showing a profession.