Fb Share not working below 50x50 images

Facebook share script minimum expecting "50x50" size to share images. So you can't share below 50px images on facebook.

Conditions details from facebook

An image URL which should represent your object within the graph. The image must be at least 50px by 50px and have a maximum aspect ratio of 3:1. We support PNG, JPEG and GIF formats. You may include multiple og:image tags to associate multiple images with your page.

Reference link

http://developers.facebook.com/docs/opengraph/