I'm resizing via css

That's generally not a good idea. Images that are too big the for target display area are a waste of resources. High speed connections often aren't. And even now, a lot of people have underpowered GPUs. Also, even a small difference in size will add up, pushing up your bandwidth usage as well as your customers.

The end result is usually much better when you tell the customers the sizes of the image areas. At very least, you avoid customer dismay when they see what your system has done to their images.