I think 5th graders could be taught the Haar transform, so I'd probably do something on that. Maybe start with a publicly available picture of Ingrid Daubechies like the one on wiki:

http://en.wikipedia.org/wiki/Ingrid_Daubechies

Then apply an averaging filter in Gimp, as well as an edge detection (difference) filter. Show both results, and maybe make something like this
http://s.eeweb.com/members/elsie_li/discussions/2012/11/10/figure-1352604976.png

The Haar transform is really easy to understand. All you really do is recursively replace a series or matrix of numbers with differences and sums. There's a tiny bit of scaling to get right, but it all comes down to the realization that 2 numbers can be represented by their sum and their difference.