Often we face issues in plugins and get error codes from CRM while debugging our server side code.
Error code 80048531 means CustomImageAttributeOnlyAllowedOnCustomEntity which is
A custom image attribute can only be added to a custom entity.
Just happened to pass by a cool resource containing the error codes and their descriptions. It is available on following path in CRM 2013 SDK:
has excel named CrmErrors
Refer to image below:
Hope it helps!