Almost any type of photo can sell via a photo library, and sometimes it's the simplest things that work best. I shoot a lot of photos for iStock and Alamy, and by far my most popular photo is a simple shot of the shiny side of a compact disk on a black background, shot at a resolution of 4256 x 2848. To date it has been purchased by well over 100 customers. Some of my other popular photos include a row of deckchairs on Bournemouth sea front, an evening shot of Stonehenge, some autumn leaves and another simple shot of a pile of carrots on a black background, all of which have sold dozens of times on a royalty-free licence. I don't make a huge amount of money from any of them, but a steady trickle of residual income is always useful.
The best way to find out what sells is to take a look at the library sites yourself. Some of them offer a service to their accepted photographers that allows you to view a list of recent search requests, so you can find out what sorts of photos people are looking for. However by far the best option is to simply shoot and submit a large number of photos and a wide variety of subjects. You may have more misses than hits, but over time you'll find out what works.