I'm the developer at sli.mg and have been asked many times if I can make our non-direct links work with RES. Obviously I can't make it happen on my end, so I figured I'd just come over and ask if you can add support for our domain to your extension.

I'm not sure what information you would need to make it possible, but I can tell you that if you append ".jpg" to ANY non direct link, you will get the source image.

I'm not sure if you handle it differently with other image hosts, but I'm flexible to make it work however is best for you. If there is anything I can do to help integrate the site into RES, let me know!

EDIT

As an edit, I just realized you accept contributors for RES. I would be willing to develop a Module to support sli.mg if need be. I've reviewed the source code and feel it would be pretty straight forward. Thanks!