How To Distinguish between a Photo and Other "Art" Images

I have posted this to both Stack Over flow and Expert's Exchange but get 
no answer:

Like many sites, we deliver a lot of imagery on our web site.

Use case: set up labels/refinements in Google Custom Search that 
aggregate and filter two different classes of image objects. Using 
Schema.org Microdata markup is very easy and works great, but we want 
the user to select either

1) PHOTOS: a "photo" is taken by with an instrument -- some kind of 
camera/lens "machine" that comprises something from "real life"

OR

2) ART: all other images that are creative works by an 
illustrator/artist: our CMS/Dbase stores these and their metadata under 
three categories, scanned paintings-water colors, scanned drawings (or 
pencil drawings even done in a computer) and vector art.

so we have four classes, which I want to group in two groups

photography
                                                             | (group 
together in searches) |

                                                                
drawings, paintings, vector

I can easily determine which is which as our web CMS gets metadata from 
the database, and drives the content through the pipe and fork dynamic 
insertion of metadata. Something like

for art: <section class="page-content" role="main" itemprop="image" 
itemscope itemtype="http://schema.org/ImageObject"> <meta 
itemprop="image" content="Red Roses" />

and toggle to this if it were a photo

"itemprop="photo" content="Daffodils" # photo

This feels wrong from a hierarchy point of view.

I would expect to have some property like "imageType" that I could then 
apply like this

itemprop="imageType" content="photo"
  itemprop="imageType" content="art"

But I don't see anything that meets this requirement in Schema.org (my 
eyes are bleeding from reviewing the hierarchy and still can't find what 
I need). There is nothing to designate "art" as such.

Any ideas?

My developer says we can use this for all non-photography images:

<section class="page-content" role="main" itemprop="image" itemscope 
itemtype="http://schema.org/ImageObject">

and for all photos:

<section class="page-content" role="main" itemprop="image" itemscope 
itemtype="http://schema.org/photo">


But this also seems out of standard because:

a) all the visuals that we offer, of all types I believe fall under 
"ImageObject"
b) But children below only include "photo" and not any other kind of 
visual "thing"
3) I could use itemtype="http://schema.org/CreativeWork"   for 
everything that is not a  photo. Though in some semantic hierarchies a 
photo, though strictly speaking is "an image of the real world made with 
a lens/mechanical instrument), may still be classed as a sibling with 
"painting" under parent "ImageObject" or parent "CreativeWork."

While I realize there is some freedom in these decisions and Google 
seems willing to parse anything i want to use, whether it follows the 
schema.org hierarchy or not... I would like to start my "career" in 
RDF/Microdata mark up (only now after doing web for 30 years!) on the 
right foot/undrstanding.

Swasti Astu, Be Well!
Brahmanathaswami

Kauai's Hindu Monastery
www.HimalayanAcademy.com