Little red x in email

I use yahoo and bellsouth .net e-mail and in some emails i receive i get blank boxes with little red "x" in the upper left corner ,where pictures or grahics are supposed to be ..clicking on it does nothing...in other internet content sometimes clicking on it opens picture or graphics...i know this is some kind of filter but have tried setting security stuff and nothing helps... i have the standard xp security stuff and norton system works too...help i get too much e-mail that i have confirmed was sent with pictures in plain view..with these hateful red x`s...they`re just ordinary pics and graphics ..not porno stuff...it` maddening...help