Marketability vs. Appearance - (textlinks vs. images)

I have been building my left navs out of images instead of text links to improve legibility and have more control over design. Does it rank you higher in the search engines if you make your left nav all text links, in opposed to jpegs that are named after the represented category and/or relevant keyword that your customers are using to find your product?

If so, can I have a textlink based left nav, and put a layer over top of it that is constructed of images so I will have the uglier but marketable nav covered up by the better looking, less marketable nav?

Or can I use an image based left nav, then have a text link based horizontal nav at the footer to attract the searches?

Just trying to keep it clean and legible, yet marketable to search engines.

This might not be the right place to post this forum, any advice on the above questions, or where I could go to find info on related topics will be greatly appreciated as usual.

Thanks.

-Bohak