How can I sync my two rotating desktop pictures?

OK, this may seem like a silly thing to be worried about, but it's driving me nuts. I just recently set up my brand new Mac Pro running OSX 10.8.2 with dual monitors. I spent more time than I should have custom creating a series of desktop backgrounds that layout on the two monitors. For each series, there is an image on the left that matches an image to be displayed on the right monitor. The idea is to utilize the handy "change picture every..." feature to have the images rotate in sync.

Now, even though the "random" button is turned off, it seems to always begin the series on a random image, then play in order. Only problem is, when the right monitor begins at a random image, it most likely is not beginning with the matching randomly chosen image on the left. I have been able to retry enough times to get them to sync, but after it falls asleep, upon waking up it chooses a new random image to begin the series.

I have labeled each image 1-7, matching the image name on the other monitor, but no difference. Has anybody else encountered this? Has anybody else solved this? Any thoughts would be appreciated.