One thing all stock agencies have in common is the necessity for keywords to be applied to the submitted images.  Each agency has slightly different requirements. Dreamstime require the most keywords, up to 80. Bigstock require the least, between 10-30. Fotolia require you to rank each keyword from the most to the least important. iStock has the most sophisticated keyword system. Before it will let you submit an image you must go through each keyword and select its meaning. Although this takes a while I can see how this information is useful for translation into other languages not to mention the accuracy of searches. In addition to keywords you must also choose the categories the image fall into as well as giving each image a title and description. One drawback of this process is all sites require it to be done as part of image submission so even if the image is rejected it needs to be done first.