Clipart and other "accessory" images are freely available in the thousands, if not millions, online. Many of the same clipart files you would find in Office suites have license agreements which grant them to be used outside of the Office suite they were designed for, and many applications, especially open source applications, have their icons, wallpapers, and more freely available online. You are likely to find many icons and pictures that match what you are looking for perfectly if you know wher to look. Be sure to look at these sites for a good start.
- KDE-Look.org - The K Desktop Environment is one of the most popular graphical interfaces available on Unix systems today. Being a popular choice for many Linux distributions, it attempts to integrate at every level, including licensing. This means that KDE's code and artwork are almost all published under an open licence, such as the General Public License (GPL), allowing anyone to potentially use, modify, or sell them, so long as they provide the same rights (and any source files used to create the originals) to anyone they provide them to. KDE-Look.org contains thousands of icons, wallpapers, and themes, many of which work well and look fitting in web documents.
- art.gnome.org - GNOME, the GNU Network Object Model Environment, is another free desktop environment for Unix based systems, like KDE. GNOME's contents are generally published under the General Public License or Lesser General Public License, allowing you to freely use, distribute, and modify all files related to it. You can find a great deal of useful artwork here, similarly to KDE-Look.org. For example, the icons you see on the menu on the left navigational menu have been taken from the Unofficial Tango and Humility icon sets. You will see that the Creative Commons and GPL agreements that these are published under allow for their even commercial use.
- deviantART - This site, and sites like it, contain individual's submissions of various artwork that they have produced including pictures, icons, and more. Be sure when examing sites like these that you are entitled to use the works in the manner that you desire.
- Classroom Clipart - This site is a good source of general clipart, primarily targetted at the production of documents and classrooom exercises, but useful none-the-less.