Many Indies operate on a shoestring. I’m one of them. If I can learn a skill and do something without having to pay for it, then I will absolutely learn to do that thing. Online graphics, coverart, banners, etc. are imaging skills that can be handy for writers working on nickels and dimes.
I stumbled on a very basic resource over at Indies Unlimited that I thought I would share. There are many other things I would write about each and my list would be longer–but if you need somewhere to start, this is a good beginning point and it lists a couple of great resources right off the bat (Canva and Pixabay).
Check out the short article here: http://www.indiesunlimited.com/free-graphics-resources-for-authors/