Seems like Dynamo would be a good tool to find missing Material images. Since Revit Live can tell us whether an image is missing, that must be accessible by the API.
Any clues on how we may do that?
I have done some digging on materials in the past and it was less fun than I had anticipated each time. If my recollection is right, then I think you have to get the material asset out of the material and then look into the properties of it. Good place to start anyway.
I believe that you can only access file names though. This post leads me to believe my memory is correct.
I guess if the image returns null you could manually review each in the Revit materials pallet. 
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Looks like Revit 2018.1 has opened up Materials.
“Visual Materials API – you can create and edit Appearance Assets – the particular instances of classes representing visual material properties – of materials in Revit. A top customer request, the opening of the materials API allows the automated creation of materials libraries and better transfer of material information between products. Now you can use any third-party tool to make changes to material finishes.”
One discussion is here: https://forums.autodesk.com/t5/revit-api-forum/revit-2018-update-1-visual-materials-api/m-p/7260170
Samples update http://download.autodesk.com/us/revit-sdk/REVIT_2018_1_SDK.msi
In the SDK sample MaterialProperties, I don’t see anything about images or rendering, only engineering data:
I suppose if any of us can figure it out, we’d make nodes that only work in 2018.1.
Yep. And I am limited to 2017 at the moment so I am officially unable to help or share in the goodies are uncovered. 