I think I have given you more than the full context! It's from the fourth edition of the American Heritage Dictionary. Under some illustrations (especially of celebrities) it says, "oil over a photograph". For example, on page 698, under a portrait of Franco. I understand one can create a portrait following a photograph instead of a live person or model. But why will one draw or paint a portrait over a photograph? That's why I am confused. Maybe this is a technical term artistic painters use for a certain kind of art work.