First, you will need to log in to your user profile and select your organization's account. Then click Action Pages in the Navigation bar.
Once in the pages tab, you can either click on an existing Action Page or click on the "Create Page" button at the top.
Then to add an image to your page, you can either do so in the Content tab by dragging the image box onto the screen, or in the Design tab where you can add a Logo image or a Cover image in the Header section and a Background image in the Page section.
With any of those options, to access your image gallery, click on the "browse" button within the image field.
You can then either upload a new image or select a previously uploaded one. You can also filter to find the name of an image in your image gallery.