Tap on the card you want to add images to and tap the 'Edit' icon at the top right corner of the screen
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Scroll down and click the 'Add Photo' button (as per the image below)

Select up to 5 images from your phone gallery
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Tap the 'Save' button to save

Tap on the card you want to add images to and tap the 'pencil' icon at the top right corner of the screen
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Tap the 'media' tab (as per the image below)

Select up to 5 images from your phone gallery
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Tap the 'Check' icon to save