Parameters are accessed with C4DAtom::GetParameter()/ C4DAtom::SetParameter(). See Parameter IDs.
Parameter: X
GV_BITMAP_XParameter: Y
GV_BITMAP_YParameter: Filename
GV_BITMAP_FN_PORTParameter: Width
GV_BITMAP_WIDTHParameter: Height
GV_BITMAP_HEIGHTParameter: Color
GV_BITMAP_COLOR