Hi Goddessophelia,
To change the image element you need to click once on the image so it becomes a selected editable block and from the floating panel (with all available options) open the library and choose another image.
If you want to change the background image, which is not a selectable image you need to click on the element which has a background image and follow the same steps.
Now, what is the name of the design you have issues with so we can take a look at it and assist you better?