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You can enhance this by getting the image file size somehow (reading ‘Content-Length’ from the response header could work), so if the 3 images loaded are only 200kB out of 1000kB total ~ 20% would be a better approximation.
Why not have a per-image loading animation? You’d only need to add the loading image centered as the background image of a img element. It’ll automagically disappear once image is loaded and you wouldn’t have a need to hide all the images before loading is complete.