Most of the image sites available at the University of Oregon through subscription (such as ARTStor) have sharing restrictions in place that say you cannot post them openly on the web. Instead of using subscription image sources, try some of the free services on this page, or take your own image.
Just like with text documents, images should be verified for accuracy. If you are not sure that the image is what it says it is, do not use it. Instead, find an image whose accuracy you can determine.
Even if an image is freely available, it still may have rules attached to it about how it can be used. Make sure to cite the image creator and any other information as required by the rights holder. Wikipedia requires that open images be licenses CC by SA (ShareAlike).