What is the final rendering of images?
If you forget alt text in images, the site will be inaccessible to those people who use text browsers, screen readers, etc.
Search engines will of course find and follow any link they can find on the page. But it won't be easy for people, especially if you're using unfriendly SEO URLs with a lot of parameters so you never know what will happen until you click on the link.
Also keep in mind that some people on a dial-up connection (there are many more than you think) have a habit of turning off the display of images on pages (to save on bandwidth since they pay per megabyte). This is the same group that actively blocks banner ads and especially Flash for the same purpose. Your kart navigation can also expose them.
I always like to treat the html document as a jagged document first and then put our semantic tags for each section. For navigation, I always end up using <ul> where each menu item is a <li> with an anchor inside.
This method is far superior to image map when it comes to accessibility and usability. You can very easily create a whole menu image sprite and use css to put it in the background.
Check this article for an example: http://stylemeltdown.com/2007/10/22/image-sprite-navigation-with-css/
more about google: http://www.google.com/search?hl=en&safe=off&client=firefox-a&rls=org.mozilla%3Aen-US%3Aofficial&hs=SCI&q=image+sprite+navigation&aq=f&oq=&aqi=g1
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Image maps can be a good choice for navigation in certain circumstances, as the alt text mentioned is important to users and search engines. Image maps are likely to impact your SEO sites even with alt text, especially if you are using server side maps (which are harder to parse for obvious reasons), it is generally recommended to avoid this problem - making sure your page has normal text links is also a little unobtrusive for your design exclusively for reading the engine. I'm sure you've seen a lot of pages that use image-based menus or graphic-card-based menus that provide a list of menu links with a little text at the bottom to make the engines easier to use and in some cases to improve accessibility.
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