Don't be nervous about messing up. There will probably be other photographers in the room, maybe even an official one for the graduating class so hopefully the students in the graduating class with share photos.
I graduated with only an AA in nursing a few years ago when I was 48, and honestly I could care less about the photos and the ceremony. What was most important was how my family had encouragement for me over the years and supported me. Those memories will always be with me, like how my husband mopped the kitchen floor on his birthday because I was too busy studying for an exam to do anything.
You sound like such a loving and thoughtful daughter, I would think just your presence alone would be so meaningful to your mother. Relax and enjoy the day! And please pass on to her how proud even many strangers, like myself are of her.