you could make an image, link it to the webpage and include in the email. They may then be able to right clk and choose to bookmark it. Any bookmark can be placed on their desktop if they want. Or make the icon for the webpage itself and when they make the bookmark that icon will be included and if a copy placed on desktop, it will be there too.
I am trying to figure out how to create/if it's possible to create a customized desktop icon (that opens a webpage) to include in an email. It is a generic, automated welcome email that people receive after submitting a volunteer application to my organization. I would like them to be able to download the icon to their desktop to help them easily navigate back to our volunteer portal. Is this possible? I would need to offer both Mac and Windows friendly.

Chowhound
Comic Vine
GameFAQs
GameSpot
Giant Bomb
TechRepublic