Beating AdBlock for Affiliate Image Ads
I suggest putting the ad image in your main image folder for one reason - if the user right-clicks the image to AdBlock it, the default setting for AdBlock is to block the entire folder where the image is located. If the user does this, it makes the site look ugly and messed up (all your images are gone) and hopefully the user will just unblock it rather than go back and try to block each individual image.
About 25% of my traffic on my main site comes from Firefox or Chrome, both of which can use AdBlock, so it's worth the little extra work to serve ads to this many more people.
You can't use this for Adsense just due to the nature of how it randomly serves ads, but for affiliate links it works like a charm.