Find erection penis Stock Images in HD and millions of other royalty-free stock photos, illustrations, and vectors in the Shutterstock collection. Thousands of new, high-quality pictures added every day. The penis may erect during sleep or be erect on waking up. Such an erection is medically known as nocturnal penile tumescence . WebMD explains the parts of a penis, how you get an erection, and how ejaculation happens.