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"Plus after 24 hours, your images are one with the blockchain - immutable at that point." - How does this work on a technical level?

right click on an image after it's been up for 24 hours and you'll see it is pulled from a steemit controlled server instead of where it was originally posted.

There's a difference between a steemit controlled server and the blockchain itself though, right? My main question is - do the images actually get stored in the blockchain?

Image is eventually a binary data, thus you can take the bites, encode them to base64 text for example, and put on block chain.
Like this you can put anything on the blockchain which can be serialized to bits.

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