This is a really interesting company Image Brief, launched in Australia 10 months ago.
Photographers only make public shots which they have selected/sold and the rest just sit dormant on their computers, this site is where photographers sign up, select genre of photography and can sell their images on request.
We, as image buyers, go to the site, put in a description of the image they want, how much we want to pay (from $200), and when we need the image by. A note is generated to all the photographers who specialise in that genre and they all submit images which they already have which may fit the brief. Then we select the image. As we have already entered a cost this is the cost of the image.
You can also brief a specific image if you want. I probably would not use this bit but I can see it could be useful for some people.
Beats using image libraries (unless you really only have budget for £10 images!)