Thanks for the reply. I was just wondering if there is a standard format used for websites.
The format depends on the image. If you have an image that has transparency, but not more than 256 color, then the GIF format is good. If you have an image that has transparency, but more than 256 colors (millions), then the PNG format is good. If you have a photo gallery, your thumbnails could be low-res lossy JPGs with links to a larger, better quality image. SVG is best for zooming without distortion.
"All major modern web browsers—including Mozilla Firefox, Internet Explorer 9, Google Chrome, Opera, and Safari—have at least some degree of support for SVG and can render the markup directly. Earlier versions of Microsoft Internet Explorer (IE) do not support SVG natively." SOURCE