One of the best sources of inexpensive stock photography, illustration, video, and audio is iStockphoto. Their database of creative assets grows every day because amateur and professional designers, musicians, and photographers upload their work and receive royalties when designers like you select and use their assets.
As above shows, a robust search function allows you to find appropriate assets by color profile, theme, and image orientation. Once you find an asset, the price is driven by its size and resolution. You can also download watermarked placeholder images so that you can see how the assets work in your web designs before purchasing.