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.
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:
An easy solution for this would be to allow users to specify different images for thumbnails vs full size images.