The method of adding these to your navigation would greatly depend on how you do your navigation. If you code your navigation yourself, you should know how to include the images. If you use a menu program, it may or may not allow images.
Implementing these with your design would be possible if you showed your design here so those with a feel for design could see how the icons might fit into the overall plan.
If I were building a site for a GP that has an X-ray room, there is a nice x-ray looking image that would fit as a navigation button.
So post some more information about what you plan and maybe you will get some really good responses.
You could also create some simple but really nice rollover effects with these. The static state could be just as they are, but the rollover could include some sort of a rectangle or square behind it (or even a reflection of some kind if you are really good with Photoshop).
Do you have a mockup of the site yet? Maybe if we can see it we can offer more focused advice.
Yes, as others have said, how to include the icons in the design depends on the layout you are aiming for. Including them in a menu might be quite different from appending them to links, for example.
The navigation is in the early stages and could be a lot better, but at the minute looks a bit messy.
The problem is leaving enough space for the text to be on the button and not on the icon. Because of this my button has lots of space on the end. Not sure how to make it all even.