Looks like they have it made in the shade, While Luna wears shades, and is the one who makes the shade.
To give just a bit of artistic constructive criticism, I feel the ground near Celestia and Luna could be put more into focus, along with the umbrella because I feel like there is no mid ground in the picture, and it took me a few seconds to realize they were on the beach and not in a ditch.
Now I can't give criticism without praise, so what I like the best on this picture is the attitude it gives. I get that feeling on Celestia's expression that she is thinking, "Yeah Luna, you're cool," in a sarcastic tone while Luna doesn't even care because she has a drink and the beach.
Gotcha, it makes sense. Just trying to leave people with more than an, "hey it's totally awesome!" stance. While praise is good, it's always good to get criticism. It's what my teachers keep telling to give and get, so I do my best to try and say something worthwhile.
Celestia and Luna should be in a buddy-cop action movie together... Solving crimes... Flirtin' with the colts... Barely paying the rent while having to deal with a sassy landlord that dispenses oddly-phrased wisdom... Box office gold, I tells ya!