In case of proxy-prefix configuration the $PATH_TO_IMAGES is not correctly converted. Removing the beginning Slash solves it for proxy-prefix configurations and other are not affected.
Merged in BjoernGruening/galaxy-central-bgruening/admin_toolshed_static (pull request #302)

In case of proxy-prefix configuration the $PATH_TO_IMAGES is not correctly converted. Removing the beginning Slash solves it for proxy-prefix configurations and other are not affected.