system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5, why do images load by first showing the entire picture distorted, then clarifying. Back in the day, they loaded slowly from the top down.
gpt
This is a product of what is called **interlacing**.  Basically, an image will store every 8th pixel first - so if you just get the first chunk of data, it'll be the 1st pixel, the 9th pixel, the 17th pixel etc.  This amounts to being a low-resolution version of the image, so it's very fast to load an image that just shows every 8th pixel.  Then you load the next line and it's every 4th pixel, then every other pixel, then finally every single pixel.  (This is the simplified ELI5 version).

Old BMP or GIF images didn't use interlacing (though interlaced GIFs are commonplace today), so when you loaded them, it just loaded the first line of the image, then the second line, then the third... which came out to the top-down loading you saw.

Interlacing only "works" if your browser supports it, and while interlacing is old, older browsers often did not support it and an area would just be blank.

Sometimes a website will cache a lower-res version of an image and serve that to you first if it detects that your connection is slow.