The best thing about selling photos online is that if you manage to create a solid portfolio of high quality images, your portfolio will become a source of passive income. You can stop working for some days, some weeks, some months, and you will still keep earning money. People buy your images while you sleep, and there is always someone on this planet who works. And who needs images 🙂 The best you can do is to ensure that they will need YOUR images. And that they can easily find them (good keywording!), and that they will find them everywhere (submitting to multiple sites instead of being exclusive to one agency). Quality matters but quantity matters even more. It might take several years to create a portfolio big enough to start making a living, but it’s time well invested.
You still need to keep your portfolio alive. If you stop submitting new images, after one month you may start noticing a gradual decline in earnings. Fresh content added by other contributors will push your images back in searches. Customers who follow your portfolio will lose interest. It is better to add a few photos every week than to add 1000 photos once every few months.
Here are some agencies to start with: