I am wondering if AdSense has thought of adding a feature for publishers to display text ads as an alternative when no image ads are available. I mean we could choose to display image ads as the first priority and show text ads when they are not available. Anyone would like to see this feature too?
That is how mine work. Go to your AdSense account, choose AdSense Setup and AdSense for Conent. In the Ad Unit just choose "Text and image ads"
Now AdSense determines to show text or images depending on the CPC or CPM. However, I want to show up image ads at first (no matter if they are high paying or not) and if there is none available, AdSense will show up text ads.
Just install adsense code on your webpage that only shows image ads. Then use text-only Adsense code as your alternative ads.