You can determine number of impressions by checking number of hits shown for the image in the Access Statistics > Images report.
If you wish to determine number of clicks, you should use a script and use it as link in the banners. E.g. such link can look like /banners/click.php?bannerid=15 and the scipt should redirect users to the destination page. In such case requests for this script (banner clicks) will be logged and you'll be able to get information on them, e.g. using the Access Statistics > Pages and Queries report.
If you wish to determine number of clicks, you should use a script and use it as link in the banners. E.g. such link can look like /banners/click.php?bannerid=15 and the scipt should redirect users to the destination page. In such case requests for this script (banner clicks) will be logged and you'll be able to get information on them, e.g. using the Access Statistics > Pages and Queries report.