All these files are 32 bit PNG images with transparent backgrounds.  Feel free to use them however you want.  If you give me credit for the artwork, fantastic.  If not, also fantastic. I don't really care. 

They were created in Adobe Fireworks CS5.  If you want to modify the original vector art, the original Fireworks PNG file is tumbeasts_fireworks.png.  

-Matthew Inman
TheOatmeal.com