Tap the Site Data icon in the lower-right corner of the canvas.

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Tap the Photo icon from the available options.
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Tap where you'd like to place the photo.
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Next, tap to take a photo or add one from your library.

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Optionally, tap the added photo thumbnail to add markup.

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Once you tap the photo the ArcSite drawing tools will appear. Once finished, tap the drawing name in the upper right when finished.