Adds stanford-only icons in image download panels

Test object: vr496qy1015

![image](https://cloud.githubusercontent.com/assets/302258/4999366/7e83ebae-6996-11e4-9bf8-9befed87841c.png)
Is this redundant when we're assigning the var like https://github.com/sul-dlss/sul-embed/pull/175/files#diff-f03b25ebba4b8f879a546ffa8221c01aR58 ?