Found under: Settings > Song actions > Edit song > [Adjust image button]
If you have an image song that needs to be rotated or cropped, you can edit the song and click on the 'Adjust image' button (only available for Android 5.1+).
You will then be able to drag the image bounds to crop it, or rotate the image clockwise or anticlockwise. When done, click on the save button.
You will then be asked to confirm the changes. You will also need to then save the song. Changes to image files cannot be undone.
You can also use this page to crop the display of PDF songs. This method is not permanent, but it does save a setting to the User 3 field, meaning this is unavailable. PDF files cannot be rotated.