what's the best way to promote this service?

So you're mainly asking a marketing question. Here on gamedev, the best way to let people know about your service is to post an announcement in Your Announcements. You could also put a notice in the Classifieds.

I would have to say the best way might be go out and contact indie game developers themselves via private message or e-mail.

But don't just spam them. Learn a little about there game and mention that in the post. Maybe even come up with a logo image for them, using art/screenshot from there game. Something personalized that shows your talent. Might take you an hour per person you contact, but there are a very limited number of indie games and you don't want to burn through them all in an hour.