A Google Groups thread asks how long does it take to get an image ad reviewed and accepted within Google AdWords.
AdWordsPro explains there is no specific date or time frame for image ads. It depends on current volume, the number of submissions, the type of ad and what day and time it is.
It is hard to give a 'typical time-to-approve' answer, since this can depend on how many ads are being submitted, the content of the ad, etc. However, I'd say 'average' might be around one to two *business* days. (This means an ad submitted late Friday is not likely to be reviewed until Monday or Tuesday, since ads are reviewed on a very limited basis over the weekends.)
Image ads require more human intervention than text ads. Search engines can easily write rules based on the words used and matched on in the text ads. But with image ads, it is hard for the search engine to see what words are being used in the image. So one would imagine it would take longer to be displayed and reviewed.
Forum discussion at Google Groups.