Issue with speed/loading images

I was constantly pressing CTRL + F5 and I didn’t get 503

but I think that since you are the owner of the domain, it would be best to use Cloudflare and use HTML instead of PHP pages
and then configure Cloudflare to “cache all” (<- an example is a little further down in the post here)
and thus in most cases it would serve all images from their servers
plus of course, other benefits that CF offers.

more info