Gogreen tutorial banner carousel images don't show up

I’m doing the gogreen tutorial and I’m at the step where I created the banner documents and now I have added them to my home page. Except the images don’t show up. They have been uploaded to the server and added to the documents and they can be seen there, but when the banner documents are added to the website in the carousel, the image source is unknown and as such cannot be seen. Although the alt is correct.