The stamps on the cover are from 2 different issues.
The stamp in the middle is from the following sheetlet which is issued on 3-August-2007: "Stamp Sheetlet to Commemorate Hongkong Post's Participation in BANGKOK 2007"
The ones on the left and right of the above are 2 different stamps from a definitive set of Hong Kong Skyline issued in 1997. The set made up of 16 of them as below:-