Improving Site Performance With Many Images

I am currently making a game in HTML/JS that includes approximately 1200 blocks per level. All of the blocks are individual images, but they are a lot of the time similar. They are

Solution 1:

A browser allows a maximum of 6-8 connection to a domain. So say you have 1200 images, it can only download 8 images at a time. This is something I learned recently. The solution to this was to create subdomains like images.website.com. That way the browser will treat the domain as if its a different one and allow you more 6-8 connections. So now 8 connections for website.com and 8 connection for images.website.come. That gives you 16 connections. Its upto you how many subdomains you'd like to make.