Truly free, high quality stock images for web use are few and far between - with that in mind, when deciding where to find images to use in your website it's time to look for something affordable, but high quality.
The absolute best library of low-cost, extremely high quality images for web use I've ever used is
iStockPhoto.
It's free to set up an account and costs $12.00 to purchase 10 'credits' (you can purchase larger packs of credits to save even more, but 10 is a great start) - credits are then used to 'buy' stock photos in various sizes from among the hundreds of thousands of options. An image's 'credit value' is based on a number of criteria including the size of the image you download (images are available in numerous sizes and for web use you can generally get away with the xsmall, or small to medium sizes = fewer credits! For example, most xsmall images are perfectly sized for web use as-is, and cost about one credit.)
iStockPhoto is a *big* site but it's pretty easy to use once you know your way around. The database of images is searchable by keyword so if you're looking for a picture of a white laptop (for example) just type that in to the search field and voila - lots of options to choose from.
They have everything from abstract images to professionally shot business scenes and everything you could possibly imagine in between.
So, if you're looking for a way to add professional looking images to your site and want lots of options to choose from check out
iStockPhoto - it's my personal favorite resource for image media.
(They also have icon sets, flash kits, and illustration!)
One additional note: be sure to read the 'license and use' agreements at any stock images site you use so you're fully aware of what exactly you are and aren't licensed to do with the images. Most stock sites allow for general web use and small run marketing materials...but check the fine print to be sure!