This tag is a bit unusual in that it does not have 2 parts. If you only use the IMG tag at its most basic, then your pages will load very slowly. The more graphics you have on your page, the longer it will take to load. That is why many sites contain few graphics. The width and height measurements are in pixels but you do not need to measure your images yourself.
Pixels are just a unit of measurement like cm or inches.
Simply right click your graphic and choose properties from the pop up menu and you will see its dimensions in pixels, usually stated as eg 80x50. The first number is the width and the second is the height measurement. Your page will load more quickly if you include the dimensions of images because it saves your browser from having to find out the size of your graphics. The last thing you should include is some text that will be displayed if the image does not appear. You could also use the ALT attribute to give some extra detail about your graphic, since if your picture loads correctly and your visitor moves their mouse over it, the alternative text automatically appears in a little cream box.