Pensieve header: Plotting the primes.

r=100; c=100;
ArrayPlot[Partition[Table[
If[PrimeQ[k],1,0],
 {k,1,r*c}], c]]

r=100; c=97;
ArrayPlot[Partition[Table[
If[PrimeQ[k],1,0],
 {k,1,r*c}], c]]