Well it depends on name of the image file and also on the alt text attribute for img tag. So never name image ad files as banner.gif, ad.gif or even site_ad.gif or 160X160.gif or any name which suggests that file contains a advertisement. Also choose alt text wisely. I have tested this method on my sidebar image which asks advertisers if they are interested in buying 125X125 spot.
When I named it nspeaks_ad.gif, Firefox Adblocker blocked it. When I named it nspeaks.gif it showed up. See how simple it is. 😉