iStockPhoto

As a division of Getty Images, iStock( short for iStockphoto) is one of the larger suppliers for stock photography and a great place to start if you are exploring stock photography as a side gig.

You can sell both images as video footage. The latter will pay better, but both have great potential for passive income. After uploading your material it will remain there for years.

You don’t necessarily need to be a photographer, as they accept (vector) illustrations and 3D renders as well.
And if you are successful in creating high demand images and footage then you may create a steady cash flow for years.

The commissions are fair and they offer statistics on your sales.
Going exclusive will get you a better deal on commissions, but you will not be allowed to sell on other websites. 

If you are new to stock photography and iStockPhoto then I recommend waiting with going exclusive (with any stock photography platform in general) until you know which one you like the most.

I tried this

I started with stock photography long ago with some pretty simple pictures taken with a low-end camera and some illustrations.
All the income I generated with it was invested in better equipment, software (like Adobe products), and trips to nice photo locations until I stopped being active as a stock photo contributor because I wanted to explore other interesting opportunities.

My pictures still remain on iStock and keep generating passive income.

So would I recommend iStock? Yes, I do. 
I learned a lot from the feedback and their community. 

Signing up

Signing up as a contributor at iStockPhoto is easy (the registration goes through GettyImages). You will however need to be approved. They will review your uploaded images and footage.
Once accepted you will find a lot of tips and tricks to approve your skills. Learning on the job!