If photography is your hobby then you can turn it into part time earning source. And if you love taking photographs then selling photos for money is a good option. There are many micro and macro stock agencies on the internet through which you can sell your photos online.
You just have to meet the quality standards set by these agencies. But be sure that you have the copyright of that image.
Even if you like to draw or edit images using illustrator or photoshop then you can sell your images and drawings on such stock photo agencies but you should have the copyrights for sale of these images incases they are edited images of some other copyright holder.
For each copy of your image downloaded you will receive some royalty for the same. If your images are really good then there is a chance that you will build a good reputation and get many loyal customers.
You can start with sites like: (click on the images to open the site)
Just register on any one of the site at the beginning to understand the process in much better manner, then later you can join more sites and start earning more.