To add your photos and pick your design, tap the "+" button at the bottom of your home screen. This pulls up the "Create a Post" screen, where you can add your photos, text, and/or a link to your website. After you've entered some text and images, you can tap the ">" in the upper right screen to go the "Customize Layout" screen where you can select your design.
You have the ability to upload images from another app on your device. On the "create a post" screen, tap "add photos" and then tap "import photos from another app." You'll have the option to then select which app you'd like to access and select the images. Please see the screenshot below.