ImageObject and Photograph Schemas

I'm working on a site that includes both blueprint images made on a 
computer and actual photographs of buildings as content. My instinct 
tells me that the former would be an ImageObject, and the latter a 
Photograph.

However, on Schema.org's ImageObject (http://schema.org/ImageObject) 
page, they have an example of a vacation photo at the bottom nested in 
an ImageObject.

I've been unable to find adequate specification on Photograph, so I'm 
finding it difficult to decide why I should use one, and not another in 
any given situation.

Are there any guidelines on the usage of these two types?