If you need to illustrate an article about software or a website, a good tip is to take screenshots.
In Microsoft Windows, a screenshot of the entire monitor, complete with taskbar, can be copied to the system clipboard by pressing the Prt Sc key. Alternatively, pressing Alt+Prt Sc will copy just the active window to the clipboard. You can then paste the clipboard into a program like MS Paint or Paint.NET using Ctrl+V, and then save it as an image file (for uploading to Wikipedia, for instance).
In Mac OS X, simply prepare your screen to how you want it to look, then hit ⌘ Cmd+⇧ Shift+3 (not the number pad 3) simultaneously. The new image will appear as a PNG on your desktop. Additionally, if you only want to capture part of the screen, you can hit ⌘ Cmd+⇧ Shift+4, click and drag over the part of the screen you would like to capture and release, also saving it as a PNG on your desktop.
When uploading, be sure to use an appropriate copyright tag, such as web-screenshot.