Herself's Webtools

First make a copy of your photo.
Second, scale it down to the size you wish to use ( Image -> Scale )

Third copy the background layer ( Dialogs -> Layers -> Duplicate Layer [duplicate layer is the button on the bottom with the two sheets of paper] )

Fourth Gaussian Blur top layer ( Filters -> Blur -> Gaussian Blur [I use 8 px ] )

Fifth Divide Layers ( Go to your Layers Dialog and under the Mode drop list select Divide )

Sixth Flatten Image ( Layer -> Merge Down )

Seventh Apply Cartoon filter ( Go to Filters -> Artistic -> Cartoon and play with the settings till you are happy with the preview, then hit OK )

On thing that makes cartoon drawings stand out from other drawings is the heavy use of black lines. Every wall, all the items in a room are given dark black outlines in a cartoon.

Or watch:
You Tube Tutorial — Gimp Cartoon