A month has passed, enough (for now) to have a look at the results of the banner-rotating-experiment.

Over the whole September, both, the sidebar ad with text and the one with image, nearly got the same amount of impressions. BUT the text ad got NO clicks while the image ad had a CTR of 0.86% (not impressive but at least there were clicks).
So, with that result in mind I will change the sidebar ad to image ads only but I will leave the php script in to still have the example working.

Yoho!

EDIT: Deleted the script and totally switched to image ads.