Knowing when an image is defined as being sexually explicit is not a clear cut matter. Some representations are obviously sexually explicit; for example pictures of sexual activity or of a naked person displaying their genitals, buttocks or breasts. An image leaving little to the imagination can generally be defined as sexually explicit!
However there are varying ideas about what is considered to be sexual and what images are sexually arousing. It is therefore important that young people know not to send images to anyone who is not trustworthy. Even when someone is thought to be trusted, there is still a risk that it could be shared without consent. A good rule of thumb is - if a young person wouldn’t want their parents to see it, they should think about not sending it.
See the esafety.gov.au website for more information.