Using existing pictures, after proper scaling, smoothing or inverting seems like fun, and is fun, but the results are not that good as one might expect. But allmost every picture makes for a good wavetable when using time mode. A good idea is to use embossing or edge detection. Drawing some simple lines or fill some space with differnt shades of grey is fun in time mode. In frequency mode one should drop power to higher harmonics.
Nice effects result from line drawing and then smothing the picture.