Webpage icons, often called favicons, must be 16 pixels by 16 pixels. Because of their small size, the design should be as simple as possible and have few colors. They must be saved as file type .ico. If you do not have a create-icon application, you may create a 16x16 image, such as .jpg, or .gif (I would recommend .gif), then convert it to an icon using "IrfanView" available for free from:
Irfan View

Finally, link to your icon in the head section of your webpage - example:

    <link rel="shortcut icon" href="weblogo.ico" />
    

You can also convert your image to an icon online at:
Favikon

For Photoshop icon plugin information:
Developer's corner - Photoshop CS4 favicon