If an advertiser chooses to run ads in image formats, Google will:
- display these ads on the Search Network
- charge an additional fee to serve these ads
- require that cost-per-thousand impressions (CPM) bidding be used
- display these ads on the Display Network
Image and Video formats are available only on Display Network.
Display is your chance to engage users with appealing ad formats. Text, image, video, or rich media formats can appear on the Display Network. Color and motion attract attention. Animation or video can tell a story. The AdWords Display Network makes it possible.
Read more here: https://support.google.com/adwords/answer/2404190