Creating a web banner in iweb?

How do I create a web banner out of a photo in iweb? I'm thinking I would just create a box the length of the page, and then insert a photo into the box. I am new to web design and am teaching myself how to do it with a lot of youtube videos and reading forums like this. Do I need to use something like photoshop in order for the photo to be high resolution quality, and to keep its dimensions when posting it as a banner say about a 1 1/2 to 2 inches long?

It doesn't need to be high resolution as 72 dpi is the standard for graphics on the web.