You will still need to upload your gallery folder to a web server somewhere.
If your current web host does not allow you FTP access to the web space that they provide, then you will need to find an alternate host for your gallery (with a web host that does offer FTP).

There are many web hosts that offer free web space that would be suitable for hosting Juicebox galleries. (Juicebox galleries do not require any special web server requirements, with the exception of Password Protection and the Download Button which require PHP 5.2.0 or later. Please see here for details.)
Try a web search with terms such as 'free web hosting' but please bear in mind that with a free hosting account, you may get little or no support, less reliability (more server down-time) and fewer features.

One you have uploaded your gallery folder, the URL that you will need to use for the 'src' attribute of your iframe is the URL for the gallery's 'index.html' file (inside the gallery folder).
For example, if your gallery folder is named 'my_gallery_folder' and you upload the complete gallery folder to the root directory of a domain named 'www.example.com', then the URL you would use for the 'src' attribute of your iframe would be:
http://www.example.com/my_gallery_folder/index.html

For reference, instructions for 'Embedding in a Web Template Site' (where FTP is often unavailable) can be found here.

Please note, however, that there are certain drawbacks to using an iframe, noted here (scroll down to "2) Using an iframe") and I would always recommend embedding the gallery directly into your web page where possible.

Unfortunately, that won't help. You need a hosting account with a regular web host which allows you to upload any files (not just images) to the web space that they provide.
Image sharing services are too restrictive. You need to be able to upload the core Juicebox files (including CSS, HTML, JavaScript and font files) as well as images and an image sharing service such as ImgBB is not suitable for hosting a Juicebox gallery.

Incidentally, Juicebox does have support for Flickr (so you can display images from a Flickr account in a Juicebox gallery) but you still need web space somewhere to upload the core Juicebox files.

The embedding code for Juicebox (found here) is HTML.
We do not provide any web hosting at all (you need to have your own web space) and FTP (File Transfer Protocol) is simply the method by which the Juicebox gallery files are copied from your computer to your web server (so that the gallery is publicly accessible on the internet).

Take a look at web hosts such as 000webhost, hostinger and x10hosting. They offer free web space which is suitable for hosting Juicebox galleries. (Please note that we are not affiliated with these web sites at all and they are just suggestions rather than recommendations. There are many other free web hosts available.)

Once you have your own web space, then all you need to do to display a Juicebox gallery on the internet is:
(1) Create a gallery with JuiceboxBuilder-Pro and save the gallery to a new empty folder of its own.
(2) Upload the complete gallery folder to your web server via FTP using a dedicated FTP program such as Filezilla (free).

That's it. You can now view the gallery on a page of its own by opening the gallery's 'index.html' file in a web browser.
There is no need to change the names of any of the gallery files and you only need to use the HTML embedding code if you want to insert the gallery into an existing web page alongside other content.

Unfortunately, we do not provide support via telephone but I am more that happy to help you out here in the forum or via email. (Please see the Support Overview for details.)

I hope my notes above help to clarify things.
If you have any further queries, please let me know.

You're welcome.
I hope you get on OK.
If you get stuck with anything or if I can be of any further assistance, please let me know.