If you want to download 400 images at a time, then it will not work even with small images, because each image to download a separate service request is created on the web-server and the number of requests is limited. For this reason, the server creates one large image, which contains all the images. Read the information on CSS-sprites for better understanding of the operation of the mechanism.
There is another option - a virtual lists. In these images are loaded as needed.