You probably used your favourite 336×280 or 25×250 medium rectangle units to show text ads. Now both these units have been included to show image ads also.
A quick look at the Adsense ad formats page will show you that 336×280 or 25×250 medium rectangle units have been included to show image ads. If you would rather not show image ads in these units, you can disable them in that particular ad code, or go to your account settings and disable image ads globally.
Google has often encouraged publishers to use ad formats which support image ads such that Adsense users can make extra money from these image ads. But some find that these image ads do not blend with their website design and text ads are more controlled in the colours they show and may help to get more clicks. Anyway, you should be seeing more image ads across the Adsense network now….