That means you're most likely using them without permission, something that gets people into legal trouble.
I would search in Google for free or use these helpful stock photo sites (paid): http://www.istockphoto.com/ http://us.fotolia.com/ http://www.shutterstock.com/ Best of luck to you!
That is an excellent source. Alongside SXC.hu I use Stockvault It's always better to be on the safe side and use stock photo than search online and just download an image to your site. Not all images online are free to download. Always check for the source and ask permission to use the image.
I've found some great images in Flickr too. You can download most of the images, but make sure to send an email to the owner and check if it's ok to use on your site.