How can I earn money through drawing/painting/graphic art design?

[Lume]

Most social media can form a base for exposure,where you can advertise and use paypal or another type of online payment service. I know Tumblr has an active art community. This method does mean you have to figure out pricing/payment/shipping on your own, though.
If your art is digital, or you have some way of making it digital (ie a scanner) you have quite a few options.
Redbubble provides a simple way to put your art on t-shirts and phone cases, and lets you decide your profits.
Society6, Zaazzle provide a similar service.
You could also join deviantArt, which sells prints and has a commission system, along with art communities and social networking. (But I've heard complaints? It's still good to have your portfolio there, even if you don't sell, as it's very organised and popular)

as mentioned by Littlefoot, etsy is a popular site for physical products.

more:
http://theabundantartist.com/15-ways-to-...rt-online/
https://www.google.com/search?q=sell+art...q=sell+art