What's He Done To You? by mudblood428. The discussion of the prompt and the first envisioned scene (which was scrapped) are great - I love to see how artists' minds work! - but the work itself is ... amazing. I cannot express how much I love this scene, or this art, and I think I may die on July 13.
What do I love about it? James' expression draws my eye first - it's perfectly self-satisfied - and Sirius' backing him up is perfect. Lily's fiery red hair is the center of the piece, fitting since she is the one Gryffindor who stands up for the Slytherin. (Interestingly, Harry is the only Gryffindor on the left half of the picture, standing just behind Snape, wearing an expression of obvious shock.) The colors, background, Snape's eyebrows... it's worth a look. Go see if you haven't already!