Adding some details to the previous response, I'd like to stress that many Wikipedia images come from Wikimedia Commons, and Commons is a different project than Wikipedia.
It is mainly a repository of free multimedia files
"that makes available public domain and freely-licensed educational media >content to all, and that acts as a common repository for the various projects of >the Wikimedia Foundation." The expression "educational" is to be understood >according to its broad meaning of "providing knowledge; instructional or >informative". 
Thus, it is possible to find great images in Commons, that come from various sources (like NASA, or other institutions like museums, galleries, libraries, academic databases, academic journals). There are great pictures on Commons.
Now, you are talking about "simple graphic", and the specific image is "meant to illustrate a basic mathematical notion". 
There are many professionals who use their free time to provide Commons (and hence Wikipedia articles) with illustrative, clear graphics.
I can't think of no valid reason not to use a graphic meant to illustrate the exact concept you want to illustrate, and meant to be used and shared for free, just because there would be the word "wikimedia" in the credits.
If the images suits your need and you feel it professional enough, please use it and give credit.
 Here's the Mathematics category.
Full disclosure: I'm a >10 years old volunteer in Wikimedia projects, and I've also been very active in movement roles like being president of a Wikimedia national chapter.