The most common formats for web images are JPEG, GIF, and PNG. These are the three formats you're most likely to come across. JPEG and GIF have been around for a long time, while PNG is a relative newcomer. PNG is a far more versatile format than GIF and JPEG but you should be aware that some browsers (such as Internet Explorer 6 and older) have problems rendering PNG images correctly, particularly ones with transparent backgrounds.
Whichever file format you use, there are three important image measurements you need to be aware of. They are dimensions, resolution, and file size.
Dimensions: This is simply the width and height of an image. Most image editing software allows you to express the dimensions of an image in a variety of units (for example centimeters, inches, or pixels...