You will need to flatten the photo on the page first. Take note though that once a photo is flatten, you won't be able to move/ resize/ transform the photo on the page.
Long tap first on the photo then select "Flatten". Then tap on "Cut/ Copy.
Select a portion of the photo. You can also drag and move the selected portion anywhere on the page.
Just hit on "Transform" to resize a portion of the photo.
Tap on "Paste" to finish the process.