The tip to enable image ads was given by the adsense team on Google University that I had the luck to be invited, check here:
http://www.contentadsguide.com/2006/...y-in-portugal/
Enabling image ads will enable video ads that is the future... I wonder I google bought youtube.com, maybe has something to do with that...
And yes, you can have image and text ads on your blocks, and thinking of this sentence : "adsense is always trying to display the best paying ads" if images show....then they are better than text ads on that moment
Google tries to place the best ads each time, but sometimes the best advertisers budget don't last long... and you may return 4 or 5 times to the same page, and you don't see the same ads each time...