I saw this lovely thread about where you can find the Cake imagery from the Portal Files.
here : general-discussion/cake-t8509.html

Does anyone know the location of the Cave Johnson portraits in the Portal 2 files?

They can be found in models\props_underground\cavejohnson_portrait01.mdl.

If you're looking for it in hammer just look for portrait in the model browsser

EDIT: Just change the skin for alternate portraits.

Magnificent! Thank you!!!

I'll be working with that model later tonight.