Actions speak louder than words, well, so do pictures.

Write what ever comes first into your mind when you see the picture.

There is "actually" no word limit but try to keep it short as possible if your text is very, very long.

http://photography.nationalgeographic.com/photography/photo-of-the-day/combahee-river-musi/

(Sorry about the picture, hope the hyperlink will suffice to check it out or on the title.)