Imgur now blocks several VPNs and have issues loading embedded previews in several fediverse platforms. So instead of using imgur, you could use one of the following alternatives for uploading your images.
https://postimages.org/
https://imgbox.com/
https://imgbb.com/
https://www.imagebam.com/
Shouldn’t this be a per instance policy? Why would the onus be on the poster?
Because pretty much all instances are being run by volunteers and hobbyists, and not a for-profit who is profiting from your content. This is just something nice to do for reducing the resources they require to run the service.