Alt text is read aloud by screen readers for people who can't see the photo, and often it's the only way they get to experience it. It should describe what's actually visible in the frame, not just repeat the dish or restaurant name (that's already right next to the photo).
Not great: "Pad Thai"
Better: "A plate of pad thai noodles with shrimp, bean
sprouts, and a lime wedge on the side"
Not great: "Restaurant"
Better: "Bright dining room with string lights and
wooden booths, a few tables full of diners"
Keep it short – a sentence or two is plenty. No need to say "photo of" or "image of," that's already understood.