In the CMS collection, you have the Projects side. It can have another name but most of the time it is called Projects. Upload the projects you want in the collection, everything is explained so that you can manage your template in an efficient way.
Sometimes images are handled in a particular way. They are not handled by the elements panel to give it background properties. If this happens, here's what to do. Go to the style panel on the right of your screen.
Look for the Backgrounds section. If it has an image, click on it.
Then change it with the image of your choice.
We advise you to look on the forums or directly on the Webflow University accessible via this link. Everything is very well explained there and they will be able to answer your questions perfectly.
For ultra customization of your template, please contact us directly at email@example.com.