The [imgur](https://github.com/kaimallea/node-imgur) API lets you upload an image (anonymously) and responds with a URL. I promisify'd that and the result is super straightforward. You don't even need to auth.
The Slack API only allows a single image URL for attachments. I was also keen on the idea of being able to swap out the card assets whenever. Unicode playing cards is a decent idea, except Slack only renders in Lato.