Can a painting be used as a current generator via absorption of photons in the paint? What happens to absorbed photons in a painting? Heat of course except for the free electrons. Could paint compounds (the dark, non-reflective) be embedded with reactive and conductive materials to generate a current?
 A: Yes indeed, though it has to have some rather special properties.
A pigment absorbs light by exciting an electron from a low to a high energy state. The question is what happens next. In most cases the energy is transferred into lattice vibrations (i.e. heat), or sometimes part of the energy is transferred into lattice vibrations and the remainder is re-emitted via fluorescence or phosphorescence. However some chemicals can use the excess energy of the excited electron to create an electrical potential.
Actually the most common pigment in nature can do this. If you Google for *chlorophyll photoelectricity", or something like that, you'll find no end of papers reporting photoelectric measurements of chlorophyll modified in various ways. However as far as I know none of these have any commercial significance.
A more realistic way to generate electricity from pigments is with dye based solar cells. See the linked article for the gory details. I don't think any of these cells have been used commercially yet, but the technology is nearly there.
I think it would be hard to turn this sort of cell into a painting, or at least not into a painitng you'd want to hang on your wall. Apart from anything else, you'd want the cell to absorb as much light as possible, so ideally it would appear black.
