A source for free imagery you may not have thought about are digital icons.

What is a pictogram?
A pictogram or pictograph is a small picture denoting a theme, concept or place. They are instantly recognizable symbols. They often overcome language barriers.

A pictogram is a form of language, of communication. It puts an idea into an illustrated form. Early pictograms examples are cave drawings.

A pictogram is a form of writing – The Chinese have been using them since 5000 BC and around 2000 BC their writing system began to develop into a more organized translation of a graphic ‘style’.

Hieroglyphs – The Ancient Egyptian writing was in practice in 4000 BC, and consisted of symbols drawn by Priests to denote the words of God. Early hieroglyphs have survived simplification or modification because they are so nice to look at.

In the offline world you see them (much larger) in different forms, such as signage in buildings like the ‘No Smoking’ sign or on the streets as stop signs, pedestrian crossings etc.

They are everywhere on the Internet and probably on your computer. In the digital world they are known as icons. You see them as program markers, folder markers and application markers like RSS feeds, Internet Explorer and Twitter.

Icons are the digital equivalent of ‘road signs’ for the Internet. Smiley faces and emoticons are also examples of digital icons.

The basic difference between a regular image and an icon is that icons are a set of graphics that have transparency and have a range of standard sizes.

That aside, there are some great icons around that can be used as clip art pieces.
There are lots of free icons. They come in a variety of file formats and sizes. As per usual check any licensing documents.

Here are my pics:

Inconspedia: This is a really nice site, well organized. It tells you how many icons in a set, and you don’t have to scramble about trying to find out who the authors are. Permissions easy to find. Nice navigation, good category list – includes cartoon, computer, animals, nature etc. Option to download png files.

Best Free Icons:
A nice selection of 20 000 + personal use icons. Contact authors if you want to use them commercially.

Icofree: Some SWEET icons here! Another collection site. Recommended.