I generally use .jpg for real life images and .gif for images such as logo, buttons, banners, etc. Rarely I use .png insted of gif images when I want them to be of high quality.
For me, I usually save photos in .JPG format at the best quality. However, for web design, I would go for .PNG instead. I hardly save in .GIF anymore since they are rather poor in quality. I have the tendency to lean towards .PNG.
Of course, jpg, it is fast, quality and easy to load. I have tried with png which is also good but I always recommend jpg. The Quality Remains same but the size shrinks down.