Despite a decline last year, so called “image spam” bounced back at the beginning of this year and has remained strong ever since, accounting for around 25 per cent of all spam, according to messaging company Postini. Pornography and stock offers are amongst the most common image spam messages.

Spam accounts for 72.9 per cent of all emails according to data from Postini StatTrack (www.postini.com/stats/). In 2005, image spam accounted for just five per cent of spam, but after seeing initial success many spammers switched tactics from text to image spam.

Image-spam has proved popular with spammers as they can easily track whether the message was opened. If it was, the image would be loaded from their remote server. Images are often coded so the spammer knows which email addresses have opened the message and are therefore good targets for further mailings.

Postini says most anti-spam products struggle to detect image spam because they are looking for word patterns. “While most anti-spam products do a good job against run-of-the-mill text-based spam, many fail totally when presented with no text to analyse, thus the rapid rise in the use of image spam.”

However, Andrew Downie, managing director at NetBop, which developed the BopSpam anti-spam software, said his company's software could “easily” detect image-spam.

“We're not really too worried at all. It's on the rise but it's quite obvious when just an image is being sent. It's very unusual that someone would send an email with only a large embedded image, so that would trigger us to investigate it further.”

The fact that the images are generally hosted on a remote website, rather than attached to the email is also an obvious clue, said Downie.

Europe's spam problem is worsening, with 25 per cent of the world's spam now coming from the continent, just behind North America (25.6 per cent). However Asia leads the way, sending 42.8 per cent of the world's spam, according to Sophos.

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