unDraw - Open source illustrations for any idea
Image library that you can easily customize. These characters might work in some scenarios, but they don't have facial expressions. Available in SVG and PNG.
Humaaans: Mix-&-Match illustration library
Customizable 2D illustrations of people. These images are licensed CC0, so they're free to use.
Jennifer Valley: Free (or Cheaper) Tool and Resources for eLearning (February 2015)
Inexpensive or free tools and resources for animation, audio, graphics, course development, and more
5 Free Desktop Image Editors for E-Learning » The Rapid eLearning Blog
Free options for image editing. I've used Picasa, Paint.net, and GIMP before, but PhotoScape and PixBuilder Studio are new to me.