I'll second the motion for iStock - they are very reliable and have great a vast collection quality pictures. I have used them for some printing of A2 posters and they came up fabulous (if I do say so myself - lol)...
We have some of Tower Bridge but that may not be much use to you.
However, I second Mike in recommending iStockphoto. Their prices are very fair and if you only want the images for your website then you only need to buy the XSmall or Small sizes meaning the cost is less than £3 per photo.
Like Mike, I would also not recommend lifting images from other websites for commercial use. As a company who provide photography and video services we too would want to protect our copyright as I am sure would any of us in business for the products and services we provide.
I know of a case recently where a company lifted and used an image which turned out to be one licensed by Getty Images. As they are so big and can afford it Getty use software to trawl the internet and look for digital matches with the images in their database. They then check if the company using the image is a customer of theirs and if they are not (which they weren't in the case I know), Getty send out an invoice which is in excess of £1,000 per image. So beware!