There are plenty of websites available that you can get free images to use on your website. Here are four good royalty free image sites with free pictures that you can download.
MorgueFile.com is a completely free stock photography site that does not require registration to download images. The photographs can be used for both commercial and non commercial use. The only downside to morguefile.com is that they do not have as large of a library as some of the other sites, but it is definitely worth taking a look.
Stock.xchng is a free stock photography website with a wide range of free images that you can download and use on your websites. You do need to register for a free account in order to download photos. You will also be required to check the terms and conditions of each image before you download. This is because some images have terms and conditions attached to them such as a link to the photographer or attribution.
EveryStockPhoto.com is more like a stock photography search engine. You enter your keywords for an image and it will search all of the royalty free photo sites for the best ones. The benefit to this is you will find thousands of images from every stock photo website all in one place.
Photogen.com has a smaller selection of images, but the images are high quality images sorted into categories for easy search. Photogen images are free for both personal and commercial use without any other restrictions apart from having to register to use the website. Photogen.com also has a sister site Texturegen.com with many free textures to use.