According to this article over at DMnews:
Image spam is down from a January 2007 high of 52% of all spam, to about 8% (as of July).
But since spammers tend to fill any vacuum they find, PDF spam is now up (along with Excel and ZIP files). They’re saying PDF files have long been trusted, business-related file types, so it’s much easier to slip past the filters for now.
Sigh. Why don’t spammers ruin the reputation of some other file type? Like Powerpoint? Wouldn’t it be great if nobody could ever send a .PPT file to you via email ever again?
More on image spam here (and how HTML email designers can mistakenly look like spammers)