Most image trading happens via peer-to-peer file-sharing
More than two million images of child sex abuse had been circulated by 100 offenders who went on to be convicted in the last 20 months, the NSPCC says.
In a sample created by analysing media reports of court cases, the charity found that nearly 50,000 of the photographs were in the worst category.
One in four offenders had held positions of trust, including as teachers, clergymen, medics and police.
A third of those involved were reported to have used peer-to-peer file-sharing.
The children's charity said sex offenders were using the software - which eliminates the need for a server or host - to stay "under the radar" and called on the next government to help combat it.
The two million photographs seized by police - some of which could be repeats of the same image - showed the "battle was far from won" on the issue, said the NSPCC's Diana Sutton.
All the cases examined for the snapshot had resulted in criminal convictions.
Ten per cent of those convicted had been hoarding child abuse images for five years or more before they were caught.
One in six of the 100 offenders involved had a conviction for sexually assaulting or grooming a child for sex, researchers found.
Ms Sutton added: "The scale of graphic child sex abuse pictures and videos over the internet is very alarming.
"The number of images seized in these cases is enough to cover the pitch at Wembley Stadium twice over - and this is just a sample.
"Many more people were convicted for possessing, making and distributing indecent images of children online during the same period.
"Children and babies are being seriously sexually assaulted to feed the demand for these photos and videos.
"And each time they are viewed, more degradation is heaped on the young victims."
In the year 2008-09 the Child Exploitation and Online Protection Centre (Ceop) received a total of 5,411 reports from members of the public and sources including industry, children's charities and law enforcement agencies around the world.
Each report "can relate to a single individual or allegation or hundreds, sometimes thousands of suspects," it says.
Ceop says the "vast majority of trading" of images takes place on various peer-to-peer platforms.
The NSPCC said there had been welcome measures in recent years to drive child abuse images from the internet, but there was still much to do.
"Making the internet safer for children should be a priority for all Parliamentary candidates during the general election campaign.
"In particular, we are calling on party leaders to show their commitment now to putting this disgusting industry out of business."
Last month probation officers' union Napo said sentencing for online sex offenders was too lenient and that many were failing to undergo proper treatment.
It warned that high risk paedophiles were not being locked up long enough to take part in rehabilitation courses.
The maximum prison term for web paedophiles at crown court is 10 years.