On Taobao or JD.com, in the picture list, the original size of the pictures is the same. How is this achieved?

I recently made a product list page. The front-end CSS controlled the width and height of the image. However, after going online, customers reported the problem of image deformation. After checking the reason, it turned out that the size ratio of the image uploaded in the background was different from the image size ratio controlled by my own CSS. .
So I looked at e-commerce websites such as Taobao, JD.com, and found that the image sizes are all of the same size or proportion

Is this uploading images in accordance with unified specifications through the background, or is there no size limit when uploading images in the background and can be uploaded at will and processed by code?