Hi @mys,
Yes this is possible but neither the imgur nor the fof upload extension support this. We, or rather @IanM, recently pushed an s3 compatible extension out that overrides this for s3. Something similar could be integrated into fof upload or the imgur extension.
I do want to point out that imgur, according to their terms, disallows uploads for use as CDN, which both fof upload and the imgur extension are doing. This means that functionality is actually breaching their terms and any images uploaded that way can be removed and your account terminated. Just so you know. Here's the source: https://imgur.com/tos and the term:
Also, don't use Imgur to host image libraries you link to from elsewhere, content for your website, advertising, avatars, or anything else that turns us into your content delivery network. If you do – and we will be the judge – or if you do anything illegal, in addition to any other legal rights we may have, we will ban you along with the site you're hotlinking from, delete all your images, report you to the authorities if necessary, and prevent you from viewing any images hosted on Imgur.com. We mean it.
