Here’s the bottom line: a picture gets an idea or point across much more quickly than a paragraph. Experience tells us that we tend to scroll past a block of text for no other reason than it was too long; we want the poster to get to the point already without significant effort on our part. There’s even an acronym for this on the very popular site, Reddit.com: TL;DR, or “too long; didn’t read”, followed by a short sentence summarizing an otherwise lengthy post. It’s the reason why Twitter’s formula works so well. It forces us to be as interesting as possible as quickly as possible.
The relationship between words and pictures in a post is very important. Text alone may be sufficient to attract readers, granted you use a compelling headline or caption, but combine it with an interesting picture and you’ve got something that stimulates the reader visually and mentally.