<img src="//www.oocities.org/chrisherron20001/chrisherron.jpg>
This is an absolute filename, used mostly when the image is not on your website.

<img src="pic.jpg">
or
<img src="images/pic.jpg">

This is a relative filename. Relative filenames are used when the image is located 
within your site. The second example demonstrates a relative filename, with the image
inside a subdirectory.

<img src="pic.jpg" alt="Alt text placed here!">
This expands a text alt text over the image when the mouse is moved over the image.