It's actually more insideous than that even. A lot of the pictures in the emails (dirty or otherwise) are "tagged" with unique identifiers that allow the spammer to know if you have seen the email even if you do not click through.
Check out the "view source" option on some of those spam messages and you'll find tags that look something like:
< img src="http://servername.com/picture0234321.jpg >
where the "0234321" tacked onto the end of the picture name is intercepted by special server software that logs a note that "the spam we sent to mlmr got read, keep sending her spam, or consider her email more valuable to sell to other spammers"