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" screen where you can select your design.
After tapping "add photos" you'll have the option to add stock images from Flickr, take a photo, add photos from your OneDrive, Google Drive or Dropbox account.
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.