I've noticed ads on my site that have only images, no words, period.
A 728x90 has 6 images in a row, maybe 120x80 each with a little space in between. Sometimes on a white background, sometimes black. With the black background these look sort of like a film strip. A similar ad runs in a 120 or 160x600 skyscraper format.
What makes me wonder about these is that thewre is NO text/words/call to action etc. They are right on target for my site though. As far as I can tell, these are not "site/placement targeted" ads.
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