Error Code 0x800700a1 when trying to add registry key through GPO

I'm trying to add a new group policy to our domain so that images are automatically sent with emails in Outlook 2013. So I created the following GPO:

This policy doesn't have any effect though. When I run gpupdate and then gpresult, the report will tell me the following regarding the policy:

Result: Failure (Error Code: 0x800700a1)

What's the problem with the policy?

The Key Path set in the GPO must not start with a \. Removing it will make the policy apply correctly.