Thanks everyone for the responses! And sorry for the confusing demo.
I work on advertising so I don't have control over the page content, just the ad (which would be the red square, and I've been tasked to develop this ad which slides the page content over, but i can't alter the page content at all.
I suspected that it was the page content that was the problem given how many images there are, but thanks for confirming that for me.
Thanks Rodrigo for the detailed response.
Mike