How can I change the fill of a shape/text box to a picture?

If the template has a placeholder image, you can drag and drop another image equal or larger in size onto that placeholder and it will be automatically resized to fit. Once you create a Text Box, or insert a shape, you can visit the Graphic Inspector and select Fill and then Image Fill. This will pop up a Finder window opened to your Pictures folder.

Select the image and press Open. You now have additional choices from the Scale to Fit and Stroke menus for the effect you seek.

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