How to add conditional sidebars

My clients site has a sidebar on each side that is populated with images that rotate using an array. It works great except some pages have more content than others so they would like it to fill with images when the page is longer or shorter. Is there a way to do that? It would even work if I could ‘assign’ a sidebar with a fixed amount of images dictated by the length of the page.

Any help is most appreciated.