It sounds like you've produced a solid, stable piece of software that's been actively used by 55,000 people. That's a comendable thing, and I don't think anyone can rightly fault you if you decide you'd like to be compensated to improve on it further.

My suggestion would be to have a premium version, but charge a minimal amount for it (especially considering your demographic). Your goal should be a high conversion rate of your existing user base at a very low price. I would suggest waiting to launch until you have a more expanded feature set though (image upload probably isn't enough, but if it supported multiple new features and multiple social networks, that should be an easy sell).