... and following on from my answer just now, there are other agencies such as Corbis, Alamy etc.
Look in your newspaper, and where there is a photograph which is not captioned (but is just put there to illustrate the article) � you will find lots of these on features pages rather than news pages � you will often find, written in tiny type around the edge of the picture, the name of the picture agency which supplied it.
Most of these agencies will charge, but some may offer some images free.
P.S shouldn't this have been in "Media"?
P.P.S. note to AB ed: actually, the whole 'adverts' heading (which strictly speaking should be called 'ads' � nobody in the industry talks about 'adverts') should be in Media, as well.