Following are excerpts from Jeld Wen describing the difference.
Picture windows are non-operational windows featuring a low profile frame to maximize glass surface. It allows a beautifully clear, unobstructed view. They can also be customized to match adjacent operational windows, i.e Single hung.
High Fixed windows are also non-operational windows, designed to let in light and add expansive views. The difference between High Fixed and Picture windows lies in the frame size. High Fixed windows have larger frames and are built to match the accompanying operating windows i.e. casement windows, for a more contemporary look.