Also, if you want to be really accurate (not sure if this applies) but the standard XHTML specs (I don't know if it appears in the HTML4.0 spec) say that you should include an alt= text for non-graphical browsers, or if the image isn't found.
For example:
code:
For HTML 4.0 Transitional:
<img src="picture.jpg" alt="This is a picture">
For XHTML 1.0:
<img src="picture.jpg" alt="This is a picture" />
Although... you probably won't have an idea what I'm on about... adding the alt= text can also help find if it's your code or the location of the picture which is in error - if there's no text or picture, it's the code, if there's text then the picture can't be found.