SEO is speculative at best.
Generally, the accepted convention is to use where appropriate and you won't suffer. For example, in your code I would write something like this:
<a href="/" title="Blahblah.com logo">
<img src="logo.jpg" alt="Blahblah logo" />
The advantage of having text instead of a logo is not much and alt + title should be descriptive, don't just put in a bunch of keywords otherwise, you will suffer SEO.