Stencil

I haven't used this in earnest yet, I'm still developing the business that I intend to use it with. But still, I'm impressed with what I see so far.
Stencil is by far the best and most convenient software I've experienced to date to create blog featured images and images for social media sharing. I'm sure it can do more, but these are the main things I was looking for, and it handles them well.
It's easy to switch to a different canvas size (HD 1920x1080, Instagram Story 1080x1920, Facebook Cover 820x360, custom size, whatever) and then adjust your text and graphics to suit that format.
I love that is has both a web app and a great WordPress plugin — I can work in whichever is most convenient at the time. And the Chrome app is icing on the cake — I imagine this will be helpful to save valuable time in my future workflow.