After reading Is CSS resizing of images still a bad idea?, I thought of a similar question. (too similar? should this be closed?)
Lets say you need to use 10 different product image sizes throughout your website and you have 20k-30k different product images, should you use 10 different files for each image size? or maybe 5 different files and use css to resize the other 5? Would there ever be combination that would be good? Or should you always make separate image files?
If you use css to resize them, you will save on storage (in GBs) but you will have slight increase in bandwidth and slower loading images(but if images are cached, and you show both sizes of the image would you use less bandwidth and have faster loads?)
(But of course you wouldn't want to use css to resize images for mobile sites.)