In the Fast Company post pictured above, you'll find some fantastic tools that can really help simplify blog-post-image creation and editing. One of the more notable ones has built-in templates to help you properly crop photos to all the various dimensions of popular social networking sites (i.e. Facebook cover photos, Twitter icons, etc.). Several help you make collages in a snap, some help you build charts and infographics with ease, and others help you make beautiful images out of quotes. All these tools are browser-based and appear to be free (or at least, have free versions available).
Take a look: 14 Tools to Help You Add Images to Your Social Media Posts (Fast Company)