Yes, you raise a great point on the Pinterest button. So, by default, Pinterest hasn't provided the button with the functionality to add pictures, which is odd. I have emailed them about this already.
There is a solution though. What we have done with Digg Digg is to allow pictures by adding a featured image. Once you do that, people will be able to pull in a picture for you. I have written a guide for you about this here: http://blog.bufferapp.com/how-to-add-the-pinterest-pin-it-button-to-your-blog-and-let-people-pin-posts-with-pictures
Hope that helps! Let me know if you have any other questions, I'll be delighted to hear from you.