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Next to nothing. I am not even sure it counts on any search engines. I use empty title attributes an descriptive alt attributes to avoid them to show on stubborn buggy browsers such as IE.
The title attribute is not used by many peoples, so why search egines should rely on it?
That is true. On Google, they count only in links. Still, it's better than nothing. Google a classic example : "logo"
You will see a lot of reputables sites that has nothing to do with "logos". The alt attribute in the link bombed them there.
I use it - but don't tell anyone ;)
You should use the alt attribute to describe an image for users who don't have images displayed. Always use it for content images so that people using screen readers don't miss important information contained in the image. Never use it for unimportant images that are just part of the page design, because users of screen readers don't want to hear endless "spacer gif, spacer gif, spacer gif..." put in a blank alt attribute thike this: alt="" instead. The purpose of this attribute was never to be used as a tool-tip but unfortunately IE incorrectly does this.
You should use the title attribute if you want to give the title of any element, images or otherwise. This should appear as a tool-tip in all browsers (including IE and Firefox). As mentioned above you can prevent IE from incorrectly displaying alt attributes as tool-tips by using title="" even if you don't need a title for that image.
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Examples of (what I consider to be) good use of these attributes:
<img src="graph.gif" alt="graph shows steady increase of widgets over time" title="Widgets Over Time" ...
<img src="photo.jpeg" alt="shaggy dog running along a beach" title="" ...
<img src="spacer.gif" alt="" ...