Found a 1/2 square hole punch on eBay: http://www.ebay.com/...0:g:1agAAOSwsN9W~gdk
True 15mm would be 19/32 or 15.0815mm which might still run afoul of the rounded edges. 1/2 inch is 12.7mm which might not be too bad, and is a "standard" size to can potentially be found as apposed to cu$tom made..
That particular one is 1.5 inches.
But yeah, I came to a similar conclusion myself this morning and decided to try a 0.5 inch hole punch. I'm pretty sure I saw one at a local craft shop in my search last week, and with rounded corners, so I'll be checking that out soon.
That said, after making some test printouts at various sizes, I personally felt 15mm was "perfect" (despite my imperfect trimming with the scissors) and that 14mm would also be sufficient, but 13mm leaves a fairly significant border around all sides of the images. I hadn't done the math yet to figure out that 0.5 inches is 12.7mm. That might be a bit too small for my liking, but you gotta do what you gotta do. . .
The alternative is to cut them all out by hand, and I'm not really looking forward to doing that with 180 or more of them.