Add image as a story element type

[shawnmjones]

Raintale already supports the link and text story element types in JSON input. An image story element type would allow users to handle images in a special way with their templates. It should not be handled that differently from text. This would just give users an additional key with which to control the look of output in templates.

A potential additional key is video. Because the user may need to other key types, this may require more thought than just adding if statements.