See Yahoo's Icons category for some examples. Be sure to check if the icons you want to use are available for public use.
There are also icons distributed with Apache, and their use is not necessarily endorsed - please be sure to follow the guidelines set forth by the appropriate group (for example, webguides). You'll probably find more attractive icons in another collection, so you may want to check the Yahoo links first.
If you do wish to use one of the icons distributed with Apache,
the path starts with /icons/.
For example, the image for a left arrow:
can be used with the following HTML code:
<IMG SRC="/icons/left.gif" ALT="Previous Document">
A listing of available images shows the different
icons already on the server. There is a description of the
icons at the end of the listing.