Yes, there are many different sources of royalty free and copyright friendly graphics and pictures. What makes ManyPixels nice is that the graphics you download all share a common look.
After selecting a color, you can search for clip art, and ManyPixels will show you clipart with the same color selected.
This means the creations from your classroom will have less of a ransom letter look and look more professional. The fact that the graphics are free to use is icing on the cake!
The graphics may be downloaded as .png or .svg, the latter are editable in vector graphics programs such as Inkscape or Adobe Illustrator.