Sorry. I should have said "complete <image> entries" rather than "imageURLs" in my post above. (I have now corrected it for others reading the post.)
To manually remove an image from a gallery, open the 'config.xml' file in a plain text editor and delete the entire <image> tag corresponding to the image you wish to remove.
A complete <image> entry will look something like this:
<image imageURL="images/wide.jpg"
thumbURL="thumbs/wide.jpg"
linkURL="images/wide.jpg"
linkTarget="_blank">
<title><![CDATA[Title text goes here.]]></title>
<caption><![CDATA[Caption text goes here.]]></caption>
</image>
(You can also delete the corresponding images and thumbnails from the 'images' and 'thumbs' folders but this is not strictly necessary to remove an image from a gallery. If the image is not listed in the XML file, then it will not be displayed in the gallery.)
In any case, this is no longer necessary.
The Juicebox-Pro download zip file has been repackaged with the fixed version of JuiceboxBuilder-Pro (the Juicebox-Pro core files remain the same) and you can download it using the link from your purchase email.
Once you install the latest version of JuiceboxBuilder-Pro, you should no longer experience this issue.
Instructions for upgrading can be found here.