Virtually all libraries allow you to choose if you would like to supply your images to them on an exclusive basis. If you go down the exclusive route it means you cannot sell those image with any other libraries. If at any point you wish to change from being exclusive you could find that you are tied into that library for a number of months until your notice period expires. If you go down the exclusive route you will receive a higher percentage commission than royalty free. Personally I made the decision to ‘pimp’ my images around to anyone who would have them and not be exclusive to any library! Stock libraries can be fickle creatures. One will accept what I think is a great image and then the next will reject it for a really random reason.
This image was rejected by almost all the libraries I supply apart from Fotolia where it has sold 145 times so far.