You can go two ways with image sourcing. You can either trawl through one of the many free image sites on the web, and take your time to select the shot which fits your needs closest, or you can opt for one of the growing number of curated image services, which do the searching for you and come up with images they think will fit a wide demographic.
Splashbase is of the latter persuasion, and offers totally free images with no real categorization, but which can fit into a number of themes. The images are uniformly cool, and apparently the list grows larger every week, but the thing that makes it work for us is the fact that you can do a pretty detailed search to find things that suit you.
So while there’s no handy Nature, Architecture or similar categories to make your search easier, at least you can home in on subjects fairly rapidly. And some of the shots are very nice indeed (even if there’s a bit of an Instagram feel to it all at times). Still and all, a solid place to start if you’re looking for some cool photos to go with your project.