Unfortunately, if you are trying to add many images to JuiceboxBuilder-Pro at once, then Adobe AIR (the platform on which JuiceboxBuilder-Pro runs) may not be able to allocate enough memory for the task at hand. Even if your computer has plenty of free RAM, Adobe AIR applications are limited to around 1GB of memory allocation (as far as I am aware).

The workaround is to add your images in smaller batches. Trial and error will help to determine a safe number of images per batch (it will depend on your own images) but, if adding 319 images at once fails, try batches of around 150 images as a starting point.

Just as a point of comparison, I have a folder of 400 test images on my desktop and I am able to add them all at once without any problem. The number of images that you will be able to add really does depend on your own image filesizes so I can't really recommend a number. Trial and error really is the best way to determine a safe number of image to add at once.

I hope that adding your gallery images in multiple drag-and-drop actions (instead of just one) is not too much of an inconvenience, although I realise that it is not ideal.