I’d suggest you look at Unsplash – https://unsplash.com/
The site has a huge archive of free-to-use-whatever photos with a large range of themes. OK, so there are a few too many “Doesn’t my iMac look pretty” shots, but the rest are great and useful for everything from website backgrounds to desktop wallpapers.
Another great one is Little Visuals – http://littlevisuals.co/page/3
The background photo for this site came from there – I liked how the various colours complimented my branding colours, and the subject emphasised the “industries” part of my logo.