It seems very strange that you work fine with JuiceboxBuilder-Lite but not with JuiceboxBuilder-Pro. I expect that both applications share similar code with regard to image processing (with the exception that JuiceboxBuilder-Pro allows multiple image sizes and watermarks).

Are you using multiple image sizes or watermarks?
If so, try using only one image size (medium) and no watermark to see if this helps.

Have you tried using the exact same images in JuiceboxBuilder-Lite that cause JuiceboxBuilder-Pro to freeze? (You can install and run both JuiceboxBuilder-Lite and JuiceboxBuilder-Pro side by side on the same computer.)

Does JuiceboxBuilder-Pro freeze on the same image every time?
Perhaps you have a corrupt image within your batch that JuiceboxBuilder-Pro is choking on. (Even if an image opens in other programs, something as seemingly innocuous as corrupt metadata may prevent JuiceboxBuidler-Pro from processing the image.)

It might also be worth temporarily disabling any security software that you might have installed (at your own risk... please disconnect from the internet first) to see if this makes a difference. Avast's Ransomware Shield, for example, has been known to interfere with JuiceboxBuilder-Pro's functionality on a few occasions (although not with the symptoms that you describe). It's a long shot but a quick and easy thing to try and if it does not make a difference, then we can at least eliminate it as a possible cause.

Yes. The latest version is v1.5.1. (You can check the latest version on the Version History page.)

No. JuiceboxBuilder-Pro is a 32-bit application. (On a Windows 10 PC, JuiceboxBuilder is installed to the C:\Program Files (x86)\ folder.)

If you try 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 usually to add your images in smaller batches.

This problem is usually encountered only when adding hundreds of images to a gallery at once.
I've never known this to happen when adding only 12 images.
Also, when this problem is encountered, JuiceboxBuilder-Pro is more likely to crash and close (rather than freeze whilst processing an image).

With this in mind, it looks like your problem might somehow be due to your source images.

Just in case it helps, how big are your source images (both in terms of resolution and file size)?
Also, what Mac OS are you using?

If possible, please zip and upload a few of your source images (and provide a download link) so that I can try to replicate your problem on my own PC.
(Please try to zip a complete batch of images that fails on your own Mac and let me know what image JuiceboxBuilder-Pro freezes on.)