Primarily, images for websites should be saved in one of two possible formats – PNG or JPG. PNG should be used for graphics and anything where a part of the image is transparent. JPG should be used for photos.
Although some mobile devices can display images at higher resolutions, for general website work make sure your images are saved at 72dpi.
If you want to create an image that matches the size of one already on your site, use your browser to work out its size. Right-clicking on the existing image will let you select Image Properties from the menu. In Photoshop, you can crop and / or resize the image to match these dimensions.