Why doesn't DCC help against image spam?

[Remco Barendse]

Now that ORDB is down my mailscanner is not filtering any spam anymore, i 
might as well disable it.

But out of curiosity, why doesn't DCC work for the image spam?

A checksum should be reasonably effective against the image spam i think? 
Assuming that they are not dynamically building each picture a bit 
differently for each e-mail that is sent?