I have to say that I think that jokes about kidnapping aren’t funny, and neither are most fat jokes. I think the sign is lame. But it doesn’t rise to the level of offensiveness.
About the only funny thing about kidnapping I ever recall reading is The Ransom of Red Chief, which is hilarious:
http://fiction.eserver.org/short/ransom_of_red_chief.html
And this is perhaps the only fat joke I find witty:
"A corpulent maiden named Kroll
Had a notion exceedingly droll:
At a masquerade ball,
Dressed in nothing at all,
She backed in as a Parker House roll. "
