We are also experiencing the same issue...
This is a known defect that has been given problem number: 6489 - "Image on link gadgets selected in Self Service is not showing in Workspaces "
If you are experiencing this issue, please log this with your support provider, referencing problem 6489.
I've been told this will be fixed in 2016.3.
I think more like you can now ALSO create Self Service dashboards in the Workspaces Dashboard Designer so they've possibly side-stepped the issue rather than fix it directly?