Adding new portfolio items

Hi, I have spent the last few days trying to figure out how to change the default portfolio images that were installed as part of the Phlox demo that I imported. From my understanding, it cannot be done from Elementor which I am using, but can only be done from the ww.wp.xz.cn dashboard.

I am able to upload the images to the media library, but cannot then create a new portfolio with these images. When I try to do this, it creates a portfolio but without an image in the preview, and doesn’t appear on the actual page (see second image).

Can you please tell me how to add new images and create a new portfolio. Any help is appreciated.