Legally, unless you have signed an agreement prohibiting such a disclosure, you may. For instance, one of your clients is called Joe’s Liquor Store, and he’s worried that all publicity is not necessarily good publicity and he’d like to “ok” any publicity before it happens. He’d have you sign a form that requires you to get his permission before connecting his business with your business.
“It is great that you are thinking about this before posting content! I would suggest first contacting your previous clients to seek their permission to use their names, logos, and pictures taken from their projects.”
So it’s not illegal, but it would be a good idea to contact these previous clients first to see if they do or not want to be advertised on your flyer. As well, certainly contact them before you use any of their Trademarked, Copywritten or Patented material.