Most of the people I've talked to think the emperor is qianlong (25 September 1711 – 7 February 1799) 乾隆帝 and the empress xiaoxianchun (28 March 1712 – 8 April 1748). However the emperor looks older than 37/38 in the artwork which was the age he was when xiaoxianchun died. So the empress could be Ulanara, the Step Empress (the second one). One thing is for sure, the empress in the artwork does not look at all like his third empress (Empress Xiaoyichun)