Hi Tom,
Are you using DoubleClick file with the Custom Schema (the dynamic profile from your Studio account)?
For Studio Custom Schema, they will only return the first data of the feed from your dynamic profile when the binding is created and initialized, as you can see it in the "Generate code" section in Studio such as the following:

You probably notice the same "HTML5 generate code" in the your file if you look at the code view in GWD. That is the reason why you always see the same image in the carousel gallery when you preview your file locally from GWD.
Can you please publish to Studio first and preview from Studio again?
By the way, in the screenshot that showed your binding "CRS | pluckandjoin Image_URL.url | join", I think when using "pluckandjoin", you probably don't need to add "join" again at the end. I tried to do the same in my own file but I have problem to preview the images in the gallery.
Perhaps you can simply use "CRS | pluck Image_URL.url | join" for the binding?
Thanks,
Emily
(Google Web Designer Team)