Re: @ Rampant Spaniel
No photograph is worth a hundreds of pounds? That depends entirely on how easy it is to recreate and how many people have done so. If you don't place that value on that work then that is fine, other people do.Many photographers would have worked with you for free given it was a not for profit project.
For what you mention yes you can get a picture for free from some sources and for very little from others, google the terms microstock and macrostock. The idea being you sell a lot at a low price rather than a few at a higher price. At the cheaper end of the market you do have to make some concessions, sometime it is in the quality (for instance, was the shot you purchased sharpened? was it sharpened correctly for the desired output medium?) and sometimes it is in the licencing terms (some free and cheap stock photography is limited in what you can use it for).
The internet hasn't made professional artists redundant. It has made the market evolve, it has opened up new business possibilities and sure there is a lot of free work out there which does have an impact but it isn't much. The prices for stock and editorial usage are set as they are because they have evolved to that, depending on your usage they can be pretty cheap, even from the big boys. There is plenty of competition that keeps prices keen, although you may baulk at a few hundred to a few thousand for a picture for a book cover, others do not. Notably in the case of books, our clients are other 'artists' who appreciate the value in the work.
So you want a picture, how much is it worth. If you had to have a trained professional with 50k worth of equipment and 20 years experience spend a day taking it, is it worth nothing? If it was worth nothing all you would have is non specific amateur work.
How strict your criteria are is likely to be the defining factor for you. If you simply wanted a picture of say a tropical beach, any tropical beach, you could probably find one for free. Now if you are a putting together an advertising campaign and you have a very specific vision of how you want the shot to look you are going to be spending in the thousands to commission a picture. Why, because you have to take a talented, insured, trained professional, have them scout a location, have them setup a shot including any props and models. lighting, scrims etc, they then have to take the shot(s), usually with the client hovering over their shoulder changing their mind every ten seconds. Then you have to go through post production and deal again with the client changing their mind (oh can you add an island in there, oh I didn't see that high ass cruise liner, can you remove it, can you add more sand please, oh I don't like where that tree move it to the left and move its shadow) and then yes it is worth a few thousand per billable hour (and a couple of days planning and post production that usally isn't billed). It all depends on your relationship with the client and how much you work.
I'm not trying to be mean, just explaining the situation may be different to how you saw it given your limited exposure to the field. Yes the internet has been a boon to some purchasers of content, especially photography, but for those of us willing to adapt, it is also awesome.