Although the clip arts appended to MS Office is very frexible and useful, someone may not have the Office softwaere.
How about the following another site?
Various photographs and illustrations created by various people are
presented here with the CC licence.
The "Attribution License" is the most frexible one in the CC licence.
Let's borrow a picture.
Earth Egg by azrainman (protected by CC Licence）
Oh, What is the CC licence?