Describe how the picture makes you feel. The most beautiful things come from the heart. For example, for a picture of a mountain I might say “What an awe inspiring image. We are blessed to witness such simple majesty.”

For a picture of a pretty bird you might comment on the fragile beauty of nature, or the grace and beauty of flight.

You can use descriptive adjectives such as stunning, glorious, incredible, touching, majestic, awe-inspiring, inspirational, heart-warming, graceful, delicate, beautiful, gorgeous, etc.