Taking it to the brink... In a situation like this, usually the only affordable way you can pry a payment from the infringers -- assuming that's what you want -- is to take them to the brink of a lawsuit and then accept a payment from them that is less than what they imagine they would be paying their lawyers to defend against you. Unless you're a savvy pro per litigant, you're going to need an attorney to do that. That's also assuming that the other side is convinced that you have a better than average chance of succeeding in court.
Just the facts ... We reviewed one of the photos in dispute and we're thinking your case is good but not a slam dunk. A court may find that your copyright is "thin" in that the originality quota is low. We know your case is different than the one we cited -- but we're alerting you that when originality is low, the photographer's position is weakened.
What is it you want? Finally, there's the question of what you want as compensation. We're assuming you haven't registered the photos (and wouldn't be entitled to statutory damages). If you won in an infringement case, you'd most likely be entitled to a reasonable license fee. For an image like yours that could range from a few hundred to a few thousand dollars. You can check with photographer friends or photo pricing books to figure out reasonable compensation.
What we would do ... In your situation, we think you should write to the infringers, tell them you own the copyright and that you want the material removed. Inform that if they continue to post and use the photos, you will consider it a matter of willful infringement. Don't threaten a lawsuit but suggest that you are ready to work something out. On your flckr pages you should also include a copyright notice with your name. At the same time you should register the photos with the copyright office (you can do it electronically). You may be able to find a low cost (or contingency) attorney via a local lawyers for the arts center if there's one in your area.