There is no way to change the imageURL prefix in JuiceboxBuilder-Pro. Images are always placed in a folder named 'images' and the imageURL paths always start with "images/". There are no user-changeable configuration options to change the name of the image folder or to set a different prefix for the imageURL. The code which handles this is compiled and cannot be modified.

If you want to use a different gallery structure to that created by JuiceboxBuilder-Pro, then you'll need to edit your gallery's 'config.xml' file in a plain text editor to change the imageURL paths.
However, if you use a text editor such as Notepad++, you'll be able to change all instances of "images/" to whatever you like across multiple files (if you have many galleries to change) in a single global search and replace action which should take just a matter of seconds.