Canva is an amazing browser-based tool to very easily mix your own gorgeous graphic design image. It provides you with well-designed layout templates, text, stencils, vectors, and free images for you to then feed into your own composition.
It is very intuitive to use as an authoring tool. It gives you lots of choices, but supplies you with enough guidance & suggestions to help you make creative choices — even if you do not consider yourself creative or designerly.
This is a great tool for creating posters, illustrations, images to use online or offline.