Trello – File Size and Storage Limits

Now there's a "Share, Print and Export" link on each card which provides a shorter URL to the card which is not tied to the board it's currently on.

For the foreseeable future, just upload the image. Or you could use an image sharing site like http://imgur.com and just put in the URL.

I think the best way for Trello to solve this would be to pull down the image from a URL. For instance, when you paste a link from any of the popular image sharing sites to the chat client HipChat, it pulls down a preview of the image so you don't have to visit it. You could put in the URL multiple times and get a copy of the image each time without creating multiple uploads.