Almost half of 13 to 17-year-olds have seen nude pictures of someone they know being shared around their school or community.
Many youngsters are sharing content with strangers online and around 70% of eight to 17-year-olds have seen content unsuitable for their age in the last year.
In a shocking report, published to mark Safer Internet Day, it was also revealed that many young people are sharing pictures they would not want their parents to see.
It gives a worrying insight into young people's digital lives - showing the extent that the internet is a part of daily life and the risks and pressures youngsters are under as a result.
The vast majority of eight to 17-year-olds questioned (84%) told researchers they had shared a photo online in the last day, with around 17% saying they had done so in the last hour.
In addition, just over one in five (22%) of all those surveyed said that someone had posted an image or video in an attempt to bully them.
Three in 10 (30%) admitted they had shared a photo they would not want their mother or father to see, while over a third (38%) say they worry about losing control of a picture they had shared online.
And worryingly, nearly two-thirds (65%) said they had shared a video or image with someone they only know online.
More than half (56%) said they have shared pictures and films on a public social media profile, with nearly a quarter (23%) saying they do not know how to control who can see their social media posts.
The findings also show that the central role of video and images in digital culture is putting pressure on children to look good online.
Young people take 12 "selfies" on average before posting one on a website, the report found - while 43% said that they worry about how attractive they look when they share photos. This fear was more common among girls than boys.
The report did find that the internet is being used by young people for positive purposes - around two thirds (67%) said that in the last year they have posted an image or video for a positive reason, such as to support friends, share something interesting or to encourage others.
Will Gardner, a director of the UK Safer Internet Centre and chief executive of Childnet, said: "It is fair to say that in 2017 the internet is powered by images and videos.
"This can magnify the risks and pressures that young people face, while also offering fun new opportunities for self-expression and creativity."
"Today's findings remind us that with an ever-changing landscape, it is more important than ever to equip young people with the skills, knowledge, confidence and resilience to communicate using images and videos responsibly and positively."
The online poll of 1,500 eight to 17-year-olds was commissioned by the UK Safer Internet Centre and carried out by ResearchBods.