The image was removed in activeadmin v1.1 and hasn't replaced for anything yet.
Another possible fix would be to include the image in the assets folder but according to this activeadmin pull request the idea was that it "should be stripped of anything outdated and to simplify" and "A new PR can decide some icons we like as a default theme that would be consistent and make sure they are easy to configure/change".
So I feel removing it is the best choice.
Before
That is with activeadmin < 1.1, with 1.1 it looks just like the after but it throws the error documented in the issue.
Fix for issue #18
The image was removed in activeadmin v1.1 and hasn't replaced for anything yet.
Another possible fix would be to include the image in the assets folder but according to this activeadmin pull request the idea was that it "should be stripped of anything outdated and to simplify" and "A new PR can decide some icons we like as a default theme that would be consistent and make sure they are easy to configure/change".
So I feel removing it is the best choice.
Before
After