You can use Flickr, but try to use the Advanced search feature and check the "Creative Commons" attribute which will only display those images that are eligible for commercial use. I think Google Images has also a feature like that in their Advanced Search setting.

Search for "public domain images" or "free stock images for commercial use" in Google and you will find lots of websites to get images that you can use on your sites freely. Though, make sure that you read the terms or license info before using any images you find on the web. And be double careful while using images of people as they require a model release. Finally, if you are not sure about what license an image has, simply do not use it.

Wikipedia images are "copy left" and all are free to use, the content is also free to use. I usually provide the source when I use it.

You can use premium services to 'purchase' the rights to use the image, some well known stock photo sites are:

iStock Photo: Royalty Free Stock Photography, Vector Art Images, Music & Video Stock Footage | iStock

Stock Photos and Royalty-Free Images by Subscription - Shutterstock

There are also free ones, such as this:

stock.xchng - the leading free stock photography site

Creative commons image search: CC Search

I use istock like other people have suggested you have to pay a small fee ,

also corbisimages.com is good and you can get images that are royalty-free