It is one of those things I am very good at, but then it is a part of my job that I hate .
When people build websites, they start off with designs. They are done in programs like Photoshop, Illustrator, Fireworks... These programs have a tool called the slice tool. The aim with slicing is to make sure that there are as few images as possible within a web page. So if for example your design is on a white background and then elements of it are standing "isolated", there is no point adding the white of the entire image to the graphics. Just cut out the part of the graphic that is not white, and leave the white part to be rendered by html/css code. That makes your page lighter and faster to download.
Your graphics program will help you do this. All you need to do is grab the slice tool and click and drag boxes around the sections of your image that you will be extracting (with pixel perfection). You can also name the slices. When you save them out, they will be individual images and will use the slice names as their file names.
Then you can start to write the HTML for the design, and positionning the images in their right places using CSS (or tables for lazy developers).
I'm a web developer