Parameters are accessed with C4DAtom::GetParameter()/ C4DAtom::SetParameter(). See Parameter IDs.
Parameter: X
GV_BITMAP_X
Parameter: Y
GV_BITMAP_Y
Parameter: Filename
GV_BITMAP_FN_PORT
Parameter: Width
GV_BITMAP_WIDTH
Parameter: Height
GV_BITMAP_HEIGHT
Parameter: Color
GV_BITMAP_COLOR