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If you create a Blank Canvas at 1650×2550 this will be the book cover at 300DPI. You can then drag on your image from the Photo Basket (at the bottom of the editor) and resize it smaller. As long as you don’t resize it larger than its original size, you will maintain a high quality, and the resulting image will be 300DPI for the book cover (including the blank space around your image).
Let us know if you have any further questions.