Using alt-text for additional SEO information
In order to tell the search engines a bit more about that image and the content, it is recommended to use descriptive alt-text in the html code snippet. But do it in moderation so your webpage is not flagged for spamming with excessive amounts of search words.
<a href="http://www.marketingpractitioners.com"><img src="URL for our black-white logo" alt="marketing definitions marketing examples marketing tips" onmouseover="this.src='URL for our colored logo'" onmouseout="this.src='URL for our black-white logo'" /></a>
As with any search engine optimization effort, make sure to research the most effective keywords for the content on your web page.
When linking an image from one web entity to another, alt-text can also draw a context bridge between the web pages or even web sites when external links are used. And as you know: every little search engine optimization effort helps.
As long as you have the users in mind and try to help the Google's of the world to understand your content and the the meaning of your on-page elements and your links.