Still Images : Web
The standard file format for web is .jpeg with a resolution of 72 dpi (dots per inch). Images are typically resized to fit on a 800x600 or 1024x678 webpage. The 'save for web' feature in Photoshop will create the standard 72dpi .jpeg and will keep the current image size.
Standard format: a .JPEG optimized and compressed for web size, resolution at 72dpi.
Tutorial: Photoshop 'save for web' 72dpi .jpeg