Images

Where text is not an option you can give spiders information on images by using an Alt tag and if the image is linked, a title attribute.

For proper mark-up and accessibility reasons all images should use an Alt tag even spacers and other design elements, in such cases the tag should be formatted as Alt="".

Alt tags have always been used to "stuff" keywords not relevant to the image itself and the practice still continues, it's not one I'd recommend as besides annoying those that use a specialist browser it is an overly overt indication that the page has been "optimised".

The title attribute is used to give information on a link, useful where no or limited link text is useable but again should only be used in moderation if link text is available.
Mouseover the logo at the top of this page to see it "in action".

To optimise images for the web rather than search engines they should be properly compressed for web use and have height and width attributes assigned to them.
Without these the page will not display until the images are fully downloaded, which if the images are large can be annoying to the viewer.

Recommended reading on optimising images:

Search Engine Watch