Why do pictures have so many formats? What are the characteristics and differences of those formats?

Different coding, bmp file is the most basic bitmap file, and its size is relatively large. Generally, the pictures you see when browsing the web are in jpg, gif or pcx format. These formats have their own advantages, but they are relatively small in size, which can speed up the display of web images and prevent unresponsiveness. If you want to study further, you can read some books on image processing, which is not clear here.