Only Internet Explorer displays the alt attribute when you hover on an image, it shouldn't. Do not use an alt attribute as a tooltip. If you need to add further information then use a title attribute:
<img src="image.gif" width="??" height="??" alt="alt text" title="more info appearing as a tooltip here" />
Visible alt attributes can be seen as unnecessary noise on your page design. Their appearance can be overridden by using a blank title attribute. e.g.
title="". This has no adverse effect on accessibility.
The company logo should have an alt attribute simply stating the company name
alt="Company name". It is considered superfluous to append it with the term "logo".
If the logo also acts as a link back to the homepage then alt="Company name: Return to homepage" would be appropriate.