Here's a selection of pictures I found that illustrate the topic of STD prevention:
This first picture, although it technically uses two words, does a good job of conveying the message that using a condom can be a way to show someone you love them. It shows that you care about them enough to protect them from and STDs. Someone who saw just this picture would be able to extract a meaningful message from it.
This image again uses a couple of words, but the combination of the picture and the phrase makes for an effective way to spread the message that sex connects sexual partners to each other.
And finally, this picture and statement do a good job of conveying the notion that unprotected sex is a gamble and its important to minimize your risks of contracting an STD. This picture combines a little bit of humor with an important message so it does a good job of catching peoples attention.