Yes that would be a violation of preemption if the parks are city owned. Several of us have been down that road. First you'll want to find the city ordinance that is banning firearms from parks. Then start a letter writing campaign. Contact the mayor and city counsel first. I had to go to a friendly local senator in order to be heard. Others have also gone to public meetings when they wouldn't respond to them. Either way it is a tedious process, but is achievable. I wouldn't worry about the pictures, rather find the laws that are being preempted and get to work.