I am using the US Flag template as intended... within another template.
The template on the profile that was reported in error:
{{Migrating Ancestor
| origin = England
| destination = Virginia
| origin-flag = Flags-3.jpg
| destination-flag = {{US Flag|Virginia}}
}}
It was the destination-flag parameter causing the error, as reported in the suggestion:
invalid image type: parameter: destination-flag
So I have replaced the {{US Flag|Virginia}} ... the profile's template is now:
{{Migrating Ancestor
| origin = England
| destination = Virginia
| origin-flag = Flags-3.jpg
| destination-flag = US_State_Flag_Images-49.png
}}
I won't know if that actually cleared the error till the next report.
I posted in G2G partly because if using the US Flag (and similar templates) as intended is going to cause a database error, that's a lot of editing to do or "False Suggestions" to mark & perhaps there's a way to exempt {{US Flag|...}} et al instead.