Do you have any non-alpha-numeric characters in your image filenames (e.g. quotes, commas, spaces, accented characters)?
If so, try renaming them using only web-safe characters.
As an image filename forms part of a URL when uploaded to a web server, it would be wise to use only web-safe characters within image filenames. Please see section 2.3 of this document for details.
Characters that are allowed in a URI but do not have a reserved purpose are called unreserved. These include uppercase and lowercase letters, decimal digits, hyphen, period, underscore, and tilde.
JuiceboxBuilder can handle image filenames with any or all of these non-reserved characters (but some other characters can cause problems).
I hope this helps to resolve your issue.