There are different theories, the practice only indicates that it is preferable to limit the animation to 8 seconds duration.
One banner can be different kilobytes (KB) in size. The size of kilobytes is most often set as a requirement from a relevant portal. The ad will be located on it. 40 KB is standard size for a banner of the 300x250 type. The file should not exceed this size.
For a banner of such size, it would be a good result to display two or three images and several vector logos. Remember well that the sum of the elements animated in the banner is not equal to the banner size. Flash animation performs calculations and can reproduce processes that add kilobytes in case of element movement.
The set of photos used in commercials is called "commodities." One of the biggest sources of such photos is called SXC.hu.
Ads that contain words (Adsense) that you can see in some banner boxes are an interesting service. It is useful both for those who want to place an ad on their site, or for those who want to advertise using this type of ads.