While teaching my son programming with Gosu/Ruby, one of the impediments I'm running into is a lack of sample graphics to use games we are contemplating working on.  Since I'm teaching him how to program, I don't want to get too bogged down with creating graphic assets ourselves, but I would like to use something a bit more sophisticated than the rectangles, triangles and circles my limited artistic talents enable me to create for us to play with.  So far, we've been working on a Breakout-like game which has simple graphics, but that will only keep his interest so long.

So my question is: Is there a good public source of graphics/sprites suitable for use with Goso to be used in games created for personal use?  For instance, sequence of characters walking, running, fighting, etc...  I'm sure they exist on the web somewhere, but I'm not even sure what Google search I would use to look for such things.

Thanks in advance for any suggestions.