very simple!
if u notice, in the picture of 13 ppl, someone is without hair, someone else without something else.....
all of those missing parts add up to create another 13th man
here's a simple tip : copy paste the pic with 13 men. select the extra man (very easy : he's the 3rd man in the column which previously had only 2 men.) now cut off his body parts and repaste them onto the remaining men replacing whatever they're missing. u will get back only 12 men