Reading over the Yola Tutorial Using the File Manager, you will not be able to upload JavaScript files if you are a Yola Bronze user:
Yola Bronze
You have 2GB of storage available per site.
You can upload individual files up to 15MB each.
You are not able to upload .js or .html files.
Even if you are a Yola Silver or Gold user, it looks like the Yola File Manager simply allows you to upload files into a container from which you can then drag and drop individual files into your web page.
It does not seem to allow you to upload a number of files (such as those required for a Juicebox gallery) and have a publicly accessible URL for each one (like web space on a regular web server).
Therefore, it would appear that Yola is an unsuitable host for a Juicbox gallery and you would have to host your Juicebox gallery elsewhere and embed the gallery into your Yola webpage using an <iframe> (documented as Option #2 here).
If you do not have an account with a regular web host which allows you to upload files and folders to web space via FTP, a suitable host for a Juicebox gallery would be Dropbox.
(1) Download and install the Dropbox client program (from this web page)
(2) Create your Juicebox gallery with the method of your choice (e.g. JuiceboxBuilder-Pro)
(3) Drag and drop the entire gallery folder (not just the contents) into the 'Public' folder within the newly created Dropbox folder on your computer
(4) Navigate towards your gallery's 'index.html' file (within 'username/Dropbox/Public/gallery_folder/'), right-click it and select 'Dropbox -> Copy public link'
(5) Embed your gallery in your Yola web page (using Yola's HTML Widget) by inserting an <iframe> whose 'src' attribute points towards the 'index.html' page within your gallery folder (the URL you copied in Step #4 above), e.g.
<iframe src="URL_to_gallery_index_page" width="800" height="600" frameborder="0" scrolling="no"></iframe>