Dear Rich: Can a city prevent me from using pictures of their workers who have the city seal logo on the workers hats and shirts. I am using the pictures in a power point for educational and critiquing purposes.The city probably can't prevent your use of the photos or municipal seals ... but there are exceptions.
The photos. If you took the photos or you have permission of the copyright owner (usually the person who took the pictures), then you would be good to go. If the city owns the copyright in the photos your use would be an infringement unless you can justify your presentation as a fair use as was done in this city ... and this one, too.
The city seal. To the extent the city seal functions as a trademark, you're free to reproduce it for editorial purposes (such as your presentation). Some city and county governments have passed laws regarding the use and reproduction of the seals. For example, the city code might read:
"The County seal shall be deemed the property of the County; and no persons shall exhibit, display, or in any manner utilize the seal or any facsimile or representation of the seal for nongovernmental purposes unless such use is specifically authorized by law."However, such codes likely violate free speech right according to a federal court and the government could only halt uses that were "reasonably calculated to convey, a false impression of sponsorship or approval [by the government]." So, under that standard, you should be fine.
Other objections? The city might object if you broke the law to obtain the pictures -- for example, you trespassed on private property, or you made use of information that you agreed to keep confidential. Outside of these concerns your first amendment rights should trump any city claims.