What type of graphic should you have on the printed on the poster? Whatever accomplishes your goals, is what you should choose. Will you use clip art, writing, or photos, perhaps you want to have drawings that you have scanned onto your computer as your graphics?
If you are a charity you may want to put pictures of whatever you are raising money for. This could be anything from an animal shelter to the blood drive. If your posters are for your church, you could use biblical pictures or Bible passages on your printed poster. You could have both a beautiful picture with a Bible verse in it. You could be advertising a Bible class and have the information and clip art.