For the limitation of storage medium and transmission speed, we usually need to reduce image file size. There are two methods in Photoshop: one is to choose a lower quality compression format when storing files; the other is to reduce the number of pixels in resample image case. Most PS users don't know the difference and connection between the two methods, and how to choose according to the practical application. Also, there is lack of research on this area at home and abroad. In this paper, these two methods are analyzed in theory and effect and it cites three real examples to discuss how to choose the method according to the practical application. It has the reality guiding significance for PS users.