Suppress images for short emails

[codinghorror (Jeff Atwood)]

The scores we were shown are

BAYES_95             3
HTML_IMAGE_ONLY_24   1.618
RDNS_NONE            0.793

Total score 5.4111

So that 30% number is right on target!

https://wiki.apache.org/spamassassin/Rules/HTML_IMAGE_ONLY_24