Backgrounds
Whenever you are running out of ideas on subjects to shoot, you can shoot backgrounds. If you take this into account you can easily get a decent portfolio of various backgrounds that could sell quite well. I tried to collect a couple of background images as the very beginning when I started to work with micro stock agencies. The first shots were quite simple - just a brick wall, water background, sand background, etc. The interesting thing is that these images are selling quite well.
My favorite background image is the desert sand background image:
To be honest, it’s not a desert. I live near a forest with this type of sand and some day I just went outside to take some photos and this was one of the series. In fact this image is my backgrounds bestseller. It does not sell so well on ShutterStock (only 11 downloads and $2.75 made), but it does sell quite good on Dreamstime.com (total made $24.50).
If you do a search on the micro stock agencies then you will see that many people are doing backgrounds. No matter how many backgrounds there are on agencies, there will always be space for some more. If I look at my stats, I must say that a decent part of my earnings are coming from background sales. Be a bit creative on this and you can get a good niche.