具有可以自定义宽、高、颜色、文本、格式等功能
https://dummyimage.com/300
https://dummyimage.com/640x4:3
https://dummyimage.com/250/ffffff/000000
https://dummyimage.com/300.png/09f/fff
https://dummyimage.com/300/09f.png/fff
https://dummyimage.com/300/09f/fff.png
https://dummyimage.com/200x300&text=dummyimage.com+rocks!