Rather than using CSS to control the sizes of images, some providers run timthumb or similar image sizing if parameters are passed to an image.
e.g.
http://hanwanggou.oss-cn-shanghai.aliyuncs.com/2017/07/A201602242271592361090.jpg This is the picture source file address
x-oss-process = image / resize, m_lfit, w_120, limit_1 / auto-orient, 0 This is the function of the host to process pictures, according to different values automatically handle the image size display.
As the following example:
http://hanwanggou.oss-cn-shanghai.aliyuncs.com/2017/07/A201602242271592361090.jpg?x-oss-process=image/resize,m_lfit,w_120,limit_1/auto-orient,0
http://hanwanggou.oss-cn-shanghai.aliyuncs.com/2017/07/A201602242271592361090.jpg?x-oss-process=image/resize,m_lfit,w_200,limit_1/auto-orient,0
http://hanwanggou.oss-cn-shanghai.aliyuncs.com/2017/07/A201602242271592361090.jpg?x-oss-process=image/resize,m_lfit,w_800,limit_1/auto-orient,0
An easy solution for this would be to allow users to specify different images for thumbnails vs full size images.