This has been an issue for quite some time:
To save you a click and some reading:
If you have a picture_entity card on the dashboard, it’s thumbnail/preview image will be a grey ‘image not found’ picture after every server restart. Every other component of the dashbaord can properly handle the server restarting and will properly reconnect / refresh once the server is back up… but image cards do not.
Why?!
As best as everybody on that thread can tell, it’s (probably) related to authentication tokens. If the issue is difficult to solve a simple way to tell lovelace to force a full page refresh on re-connection would be appreciated.