No it is neither spam nor white hat. It is REQUIRED. All IMG tags MUST have ALT (which may be empty if the image is just a logo etc...) e.g.
<img src="path/logo.jpg" alt="">
Note: Strictly speaking, a logo is not content, so it should be a background image in the CSS. All images in the HTML should be content. As such, they should have a description in the ALT, as in:
<img src="path/windmill.jpg" alt="the windmill theater">