Select the type of materials you'll want to use to illustrate your theme. Imagination is the key here. You can use photos based on the theme or draw images related to your theme right on the poster board.
Arrange the pictures or drawings in a manner that's easy to read or to understand. If you're doing a sports theme, the logo or mascot of your sports team will be a good image to put at the top of the poster. If you're doing a poster for a blood drive or for donations for a fundraiser, a picture of a red cross symbol or a picture of someone who will benefit from the fundraiser are great images to use. Leave a little space on the top or bottom to write in a title if you choose to do so.
Use pictures, images or drawings in a collage style if you want to add a bit more creativity to the piece. Find lots of pictures or drawings related to your theme. To do this, look in magazines and periodicals and cut out the images, then paste them onto the poster. Fill up the body of the poster with the pictures and slightly overlap them onto one another to create a collage effect.