Cohen’s d is mean/stdev.
This is an easy calcualtion in imcalc - select the mean and the stdev images, so
i1 = mean
i2 = stdev
The basic formula would be just
i1./i2
However, that causes an error with stdev of 0, so use this formula to set effect size to 0 where stdev is 0:
i1./(i2.*(i2>0) +999999.*(i2<=0))
Strickly speaking, it’s not 0, just 1/999,999.