Now I will let you into the little secret that I have discovered:
Having interesting photos brings visitors to your blog too. A lot of people come to my blog after having looked for an alien picture. And that alien post is my most popular post and has had thousands of page views by itself.
And that is the whole tip. To get more visitors, choose an interesting, relevant, popular picture and that will lead people to your website as well, after that it is up to you to grab their attention and keep them there.
Cute kittens always help too!