Hopes Father Will Buy Her Picture -- Frances Quashie-Idun, 21, brought a gay West African costume to wear at parties and dances when she came to Britain from her home at Accra, Gold Coast. But for most of the time she has worn the costume she has been sitting quite still while artist William E. Reid painted her. The portrait is hanging in the Piccadilly, London, exhibition of the Royal society of Portrait painters. Frances, studying English at University College, Oxford, hopes her father a Gold