All images (and other elements that require it) must have an alt attribute. If the image does not require an alternative representation (alt text), use an empty alt attribute like this...
<img src="file.gif" width="100" height="100" alt="">
That's two quote marks side by side.
What I've found over the years is that when you design with CSS and minimize/eliminate the use of tables, you effectively reduce your code bloat by eliminating the need for all of those spacer gifs that do require an empty alt attribute to pass validation.