Visuals set the stage before people even walk in the room. When people are greeted by a fun and friendly poster, it makes them feel welcome. And puts them in a more positive mood. They know, before anyone has said a word, that you've put some thought into the gathering...and this gathering might actually be fun!
Your visuals don't need to be super elaborate. Simple can be fabulous! Just stick with one main image and a short title. If you need a subtitle, make sure it's short and put it on the bottom, under your image. Add some color and you're set!