Thank a lot for the suggestion!
Unfortunately it looks like the Imgur API requires the use of OAuth which currently the Spice instant are unable to support/implement. If they were to allow the use of API keys as an alternative form of authentication, then an Imgur Spice instant answer could definitely be created.
We often suggest our developers reach out to the the API providers to see if they can make an exception, so you're welcome to ask Imgur if they plan on supporting the use of API Keys as an alternative.
For what it's worth, images are definitely something we have in mind, but in the meantime DuckDuckGo users can use our bang syntax, e.g. "grumpy cat !bi" which in this case, will perform an image search on Bing.
More info here: https://duck.co/help/features/images