It is good to use Imgur API to host images which are posted by the user. It is used by various companies like StackOverflow and NodeBB.
This would require a plugin and has certain drawbacks cause third party free services have a habit of nuking images over time.
please develop plugin for this, that would be awesome, it helps us to reduce server storage!
We have no plans to build this, if you would like it built perhaps do a post on #marketplace
You may want to consider amazon s3 for uploads which is already supported by discourse.
but imgur is free for all