Spam using images rather than text now accounts for most junk email, according to McAfee.

The change began in late 2005 to counter aggressive spam filtering which blocks emails containing certain words or phrases. By last October 40 percent of spam was image-based, and by the end of the year the proportion had risen to 65 percent.

As an image spam is typically three to four times the size of the average text email, it is taking a significant slice of people's bandwidth - and some users pay by the gigabyte.

McAfee says one of the problems it and other security companies are facing in the image-spam battle is that analysing the content of an image is CPU intensive since optical character recognition (OCR) is required to pick out individual words.

The task is made even more difficult because in most cases each image contains random background noise or has a random file name, making it unique. Animated gifs and multi-layer image files are even more advanced techniques send unsolicited messages across.

McAfee's claims its anti-spam doesn't analyse images because of the processing power but instead looks at the source of the email and a summary of the contents of the e-mail.

It states its detection rates are 99 per cent or more, but doesn't say how many false positives it gets using this method.