farmboy - 11:37 am on Oct 31, 2010 (gmt 0)
We've found that our publishers who show both image and text ads are able to earn higher revenues. Opting into both text and image ads increases competition among the ads...
This seems to indicate that the increase in revenue experienced by some is not that image ads necessarily perform well, but that the increased competition for the ad space results in the revenue increase.
In theory, I guess a publisher could enable both image and text ads and experience an increase in revenue without having much of an increase in the number of image ads shown on the site.
This may also at least partially explain why Google gives us 2 universal options (in addition to individual ad options) - We can enable account-wide (1)text ads only or (2)text & image ads. But we can't enable image ads only account-wide.