JPEGs are really good at throwing out the right data for photographs. GIF and PNG are non-lossy formats; you keep every single pixel you put down (although for

JPEGs are really good at throwing out the right data for photographs. GIF and PNG are non-lossy formats; you keep every single pixel you put down (although for GIFs you do lose fidelity in the color space if you use more than 256). JPEGs will do a really bad job with pixel perfect, but faces/mountains/flowers/... will look nice even compressed very heavily.