image spam is a form of spam where the contents of the spammed email are generally little more than an image file containing an advertisement for the product being spammed... often such emails don't even include a clickable link to follow to get to the vendor's website in order to buy the product - instead they contain a url in the picture that the recipient then has to type into his/her browser in order to get to the vendor's site...
a large amount of the spam seen lately is image spam... martin overton has posted about it on a number of occasions on his blog and this post on spam in particular shows multiple types (i think the ransom note format is rather humourous) though a lot of advancement has been made in spam obfuscation since even that short time ago...
the basic premise behind using images in spam instead of normal text or html is that it makes it much harder to analyze in an automated way... essentially, it uses the principles of CAPTCHA in order to foil anti-spam technologies... it used to be that an anti-spam technology would simply look at the contents of an email to tell if it was spam or not, but with image spam it becomes necessary to create software to extract the text from an image (often a distorted image) before those contents can be analyzed and what makes CAPTCHA work is that that's not easy for a program to do...
this isn't the first time such dark implementations of CAPTCHA have been seen... certain email worms have used it in the past in order to foil automated email scanners (the worms sent themselves in a password protected archive along with an image containing the password - and yes, even with all the hoops one would have to jump through in order for such a scheme to be viable, a number of those worms were successful)...
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