When finding stock images for your website you need to be aware of image licences. Image licensing has a number of facets, you have licenses for specific uses, usually from photographers, you have royalty free licenses which are basically “do what you want” from many stock photo websites, and you have the uncertain world of Creative Commons & Attribution licenses, and the Public Domain which can cover anything from historical records ant out of copyright antiques to NASA photos of the universe.
The US Government has a clear certification for Public Domain and Attribution licensed resources – since they mainly for educational use, but no reason why you can’t use them in your website. Did you know that almost all NASA images are free for you to use as you wish with only a credit required? There is a law firm in Southeast Asia that used NASA satellite photos of the countries around the region where it is based as a core part of its company branding, a really nicely executed concept with no more than inventive sourcing of imagery.
Flickr is often used as a source of imagery, since many photos adopt a Creative Commons license to allow sharing, reproduction, and creative work derivatives. This makes it a great resource, but most images are not licensed for commercial work – this means using them on your company website is completely out, unless you can contact the photographer and ask permission or for a quote. Be careful of derivative licenses, when you use someones Creative Commons Attribution-Share Alike licensed file in your website, technically you must distribute your work under the same Share Alike license – basically don’t use them in a project you want to charge for.
Wikimedia Commons is great for some of the weirder stuff, historical and scientific, any image that is used in the Wikipedia Encyclopaedia is contained in the Commons – a repository for all fair use and public domain images used and offered as a resource. Do a search for “public domain” and go crazy, you can do what you like with these images. It takes a while to sift through and collect a usable set of images, since there’s a lot of random stuff by nature of it being a public encyclopaedia resource. You can find some gems here though, old maps of your city or country, go for a vintage theme, find some old postage stamps, postcards and photos that are out of copyright. Many people post great photos on here under public licenses, so you can grab them too. Please remember to attribute the author where required, and even if not required – you have got free imagery that someone had the inspiration to create and then let you have for free, the least you can do is credit them where you use the image. The Commons is also great for country and international resources, most places around the world are covered – although sometimes with only average quality photography but you can’t complain. Check out Picture of the Day archives for some stunning shots, they vary in license restrictions though so select carefully and don’t use an image that you don’t have permission for – there are plenty of alternatives that will fit.