In order to display images in a Juicebox gallery, the images need to be uploaded to a web server somewhere.
Many users have their own web space where they can upload their gallery folders (which include core Juicebox files and images).

You could perhaps sign up for a hosting account with a regular web host which would allow you to upload your Juicebox galleries to your own personal web space using a dedicated FTP program such as Filezilla.
There are many web hosts that offer free web space which would be suitable for hosting Juicebox galleries.
Try a web search with terms such as 'free web hosting' but please bear in mind that with a free hosting account, you will likely get little or no support, less reliability (more server down-time) and fewer features.

Also, you could host your images on Flickr.
Juicebox can be configured to display images from a Flickr account (using tags or sets to display only certain images).
For reference, the Flickr options supported by Juicebox-Pro can be found here.
You can quickly and easily create a Juicebox gallery sourced by Flickr images using JuiceboxBuilder-Pro.
On the 'Images' tab, choose 'Flickr' as the 'Image Source', enter your Flickr Username or Id and click 'Load Images'. You can then configure your gallery as required (on the 'Customize' tab) and save it (on the 'Publish' tab).
Please note that you will still need to upload the core Juicebox files (as well as an HTML page containing the gallery's embedding code) to a web server somewhere but the images will be taken care of by Flickr.

If you do not have any web space at all, then you could host a gallery on Google Drive following the instructions here.

Take a look at the Getting Started support section which should help you install JuiceboxBuilder-Pro.
You could then quickly and easily create a Flickr gallery by going to JuiceboxBuilder-Pro's 'Images' tab, selecting 'Flickr' as the 'Image Source' and entering your Flickr User Name (or Id).

If you do not already have a Flickr account, sign up for one (its free) and upload a couple of test images before trying out JuiceboxBuilder-Pro.

The Juicebox plugin for WordPress can be downloaded from this web page. Installation and usage instructions can be found on the same page.
If you get stuck with anything specific, just let me know and I'll try to help you out.