But Snapchat has denied that its service was ever compromised.
'We can confirm that Snapchat's servers were never breached and were not the source of these leaks,' Snapchat told MailOnline.
'Snapchatters were victimised by their use of third-party apps to send and receive Snaps, a practice that we expressly prohibit in our Terms of Use precisely because they compromise our users' security.
'We vigilantly monitor the App Store and Google Play for illegal third-party apps and have succeeded in getting many of these removed.'
One poster has claimed that the full leak will take place on 12 October, and will be uploaded to a searchable database, which suggests the images will be linked to Snapchat user IDs.
The warning was first spotted by blogger Kenny Withers, who described it as 'one of the largest data breaches of online profiles.'
'To put this into perspective, just imagine the celebrity nude scandal,' he wrote.
'Now replace iCloud with Snapsave and blow this up by hundreds of thousands of accounts. Now you have the Snappening.'
To prove that he or she had hacked a third-party app, the poster provided pictures allegedly from Snapchat.
But some users have highlighted that they can find the images elsewhere.
While Snapsave has been named by some as the third-party app that was hacked into, an anonymous photo trader told Business Insider it was in fact SnapSaved.com.
The news comes just weeks after hundreds of celebrity nude images were leaked online through 4chan, following a hack of Apple's iCloud in an event dubbed 'The Fappening.'
This isn't the first time Los Angeles-based firm has faced security concerns.
Last year, developers launched a SnapHack app that let users save and re-open Snapchat messages, without the sender ever knowing.
The app followed an announcement by a forensics firm in Utah that discovered a way to find the so-called deleted pics, and created a way to download them from the 'hidden' location.
In May, Orem-based firm Decipher Forensics discovered that Snapchat was saving the images.
And the group said the app's code suggests that it isn't a bug; the developers appeared to have coded the app to do this, despite claiming that all images are automatically destroyed.
Richard Hickman, a digital forensics examiner from Decipher Forensics said: 'The actual app is saving the picture.'
'[Snapchat] claim that it's deleted, and it's not even deleted. It's actually saved on the phone.'
- Daily Mail