Based on bilateral diagonal stretch,this paper proposes a new invertible two-dimensional map which can be used in image encryption. The map consists of the left map and the right map, which can encrypt images by processing image stretch-and-fold. The process of the left map is that a square image is stretched respectively by diagonal firstly. Pixels of each column can be inserted into pixels of each horizon. Then, the original image can be stretched to a line. Finally the line is fold over to a new square image whose size is same as the original image. The right map is from right to left. The left map and the right map shuffle the positions of image pixels which are invertible. This paper formulates the algorithm of the map,designs a method of key generation and analyses the security of the proposed image encryption. The simulation results show that the proposed encryption approach is valid.