If you can edit an Actionscript file and recompile the SWF, then here's what to do.
1. Open StageManager.as
2. Find the function init()
3. Change the line
"mCurrentImageIndex = 0;"
to the number you want to start with. Remember that this is an array, so the first image is "0", then second image is "1", etc. And don't go past the (total number of your images) - 1.
If you use buttons, you'll want to change the following in StageManager.as init()
If you want to go a step further to make it easier, you can add a global variable in Option.as ("static var startImage:Number = 1" and replace "mCurrentImageIndex = 0;" with "mCurrentImageIndex = Options.startImage;"