AdSense keeps "pushing" rich media/image ads on publishers at an alarming rate of change. Just reviewed July to August MTD and AdSense has increased my share of rich media/image ads on my top location by 30%. The placement is such high traffic that anytime I switch to just text only, RPM tanks immediately. If you run all ads types my guess is that you are seeing the same trend. Earnings affected (down 20%) as a result of this shift by Google.