Navigation

Key Objective: Usability

Image Nav links
The image bar is often used for the first-level navigation, because these are more fixed.
Horizontal bars are appropriate where the number of items is known in advance, there are not going to be any more items added, and there is enough width to accommodate all items safely in the target screen resolution (generally 800px).

Side image nav bar is better when the number of links will exceed the width of the page.
Benefits
It is relatively familiar and intuitive (provided it is presented in a conventional format). Can provide high end aesthetics and not sacrifice usability.
Drawbacks/risks
Not easily updated (more expensive to maintain). Search engines only read text so images can be unfriendly to search engine robots.

Horizontal image top nav bars:

Text Nav links
This has become a common navigation mechanism over the past few years; it is uncomplicated and has few usability challenges.
The principle is that you click on a word and the appropriate page will appear. This page can also have additional links for the sub levels of the site.
This device has a limited number of problems, is frequently less elegant a solution BUT search engines love this. It is text.

Benefits
It is relatively familiar and intuitive (provided it is presented in a conventional format). It can provide relatively simple access to a complex structure.
Drawbacks/risks
Now we are back to aesthetics less elegant. Simple text is often not what you picture when you think of your website.