Rob,

I think you need to provide a bit more information. Is each image already thresholded so that significant voxels are 1 and the rest are zero? If so, you can combine all your maps into an overlap map that shows the sum intensity at each voxel:
  Draw/Statistics/CreateOverlapMap
If you want to do something a bit more complicated, you can modify my Matlab script nii_mean_stdev (from Github) to threshold and combine images.

-c