Here are some options.
Compute and plot the maximum value at each pixel:
imagesc(max(a100,[],3))
Compute and plot the minimum value at each pixel:
imagesc(min(a100,[],3))
Compute and plot the sum of all the images:
imagesc(sum(a100,3))
Compute and plot the average of all the images:
imagesc(mean(a100,3))