Powerapps "IsAddImage" property in "Site Inspector" example

Based on the "Site Inspector" template I try to create my own app. However I get stuck on the "IsAddImage" property.

In the Site Inspector template I can use is but my own solution does not recognise this property. I can figure out where is comes from and what is actually is.

I try to use this function in my solution "If(IsAddImage,Camera1.Photo,Gallery1.Selected.Photo)" but, as mentioned, I get "the function has some invalid arguments" which is the "IsAddImage". Can someone help me out here?

(it is in the EditImageScreen)

Thanks, Mike