the files are keep in MetalicConductor/PositiveParticle.gif and NegativeParticle.gif

so %images% on the imageset is the way to call ejs to draw images in array.

and in the initialize page for init positives

int sqr = (int) Math.sqrt(positives);
for (int i=0; i < positives; i++) {
 images[ i ] = "./MetalicConductor/PositiveParticle.gif";

i also don't understand fully, but hope that helps