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Ok seems now you can upload directly your pictures to Steemit with the "Drag and drop or select item" in the editor when you post.
This article is one year old, and apparently it wasn't possible at that time. Now it is.
Proof in 2 of my posts :

https://steemit.com/art/@eternalsunshine/a-nice-black-and-white-ink-pen-hand-drawing

and

https://steemit.com/colorchallenge/@eternalsunshine/4eseup-color-challenge-yellow

So, no need to host externally :)

thank you :)

good to see it should be possible however i cant seem to get this to work for me yet

Almost all my images until now were posted this way. Just drag & drop your image in the text box while editing and it will automatically upload it to a Steemit server and insert a link to the image.