Help! Pinterest button problems

Hi, I′m having a problem with the Pinterest button. It shows randomly on some of my images, and on others it doesn′t appear. Also, if I click to pin on the images it does appear on, the pinterest popup window comes up, but the area on the left where the image should appear is just blank. I can only see the title. When I try to pin it to a board it says “Image is too small” and won′t pin it. The original image on the page is 600px x 600px. Please help! Thanks