Size hi res icon correctly

Stops [this happening](https://s3.amazonaws.com/prod.tracker2/resource/22301288/photo.PNG?AWSAccessKeyId=AKIAIKWOAN6H4H3QMJ6Q&Expires=1377789795&Signature=A5gKhUupBc%2FFADb921J3vm7L3JA%3D) on retina displays