Laws covering so-called revenge porn are not fit for purpose and police still need more training, experts say.
Victims should receive anonymity and laws need to include threats to share images, according to Sophie Mortimer from the Revenge Porn helpline.
Figures from 19 forces in England and Wales revealed police investigations have doubled in the last four years but the number of charges has fallen.
The National Police Chiefs Council said forces take the crime "very seriously".
Revenge porn - the sharing of private or sexual images or videos of a person without their consent - became an offence in England and Wales in April 2015.
Similar laws were later introduced in Northern Ireland and Scotland.
Figures from 19 of 43 police forces in England and Wales show the number of alleged cases being investigated by officers has more than doubled in the last four years - from 852 in 2015-16 to 1,853 in 2018-19.
However, the figures also reveal that the number of charges dropped by 23% - from 207 to 158 - during the same period.
Revenge porn is currently categorised as a "communications crime", meaning victims are not granted anonymity.
In the last year, more than a third of victims decided not to proceed with the case.
Some say it is because they are not granted anonymity, while others cited a lack of police support.
In October 2016, Alice Ruggles, 24, was murdered by a former boyfriend who cut her throat after breaking into her home in Gateshead.
After her death it emerged that her killer, Trimaan Dhillon, had threatened to share intimate images of her online as part of a campaign of stalking and harassment.
Alice's mother, Dr Sue Hills, said threatening to share images should be made part of the law.
She said her daughter may have sought help sooner if Dhillon had not held the threat over her.
"It causes immensely serious psychological damage - it is a crime," she said.
Ms Mortimer, from the Revenge Porn helpline, backed her call.
She added: "We'd also like to see it made a sexual offence because that would guarantee anonymity for victims."
She also called for better training of police officers.
"It's all very well changing the law and making these things illegal, but if the frontline services don't understand what the law actually means then you've only done half the job."
Research by the University of Suffolk found 95% of police officers who took part in a survey in 2017 said they had not had any training on revenge porn legislation.
A joint Ministry of Justice and Home Office statement said: "When we engaged with victims and campaigners in designing the new law they accepted that the motive for this crime is almost always malicious, rather than sexual, which is why the law considers it a non-sexual offence.
"We launched and continue to support the Revenge Porn helpline, which helps victims to speak with the police and to social media companies about removing the content."
Chief Constable Simon Bailey from the National Police Chiefs' Council said forces "pursue all lines of inquiry and prosecute people where appropriate".
"The College of Policing has produced a briefing and training note, which all officers involved in these types of investigations can access."