Anytime you add an image, you can add it directly to the image library to reuse in other documents and templates. This can be done either through the image block or page settings (for setting the image as page background).

Before uploading your image, check the "Automatically add image to Image Library" setting and that's it!
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You can also access the "Image Library" section on the left panel and upload your image from there.

Click on the Insert tab on the top left panel, select Image content block.
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Select the Add image option on the block to upload your files.

Then select "Image Library" on the left, and there you can find your image - preview it or simply click on it to add to your document.
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If you'd like to set this image as the page background, click on the three horizontal dots button at the top of the page to access page properties. From there, upload your file under "Background image".
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Click on the Insert tab at the top of the page. Then select Add from > Image library.
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Add an image block to your document, then click on it and select "Image Library" on the left. Once you are done with managing images, you can delete the image block.