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Right-clicking is disabled within a Juicebox gallery but, in addition to showOpenButton (as noted above), there is now a direct download button (introduced in v1.5.0 - see the Version History page for a full list of changes).
Set showDownloadButton="TRUE" (in JuiceboxBuilder-Pro's 'Customize -> Button Bar' section) and the Download Button will be displayed on the gallery's Button Bar. Clicking this button will automatically initiate a download of the current image (without the need to open the image in a new page first).
You can check out the Download Button in this demo gallery (2nd icon from the left on the Button Bar).
By default, the Download Button will download the imageURL.
However, if you use a Multi-Size Image gallery and use large images, then the Download Button will download the largeImageURL
If you assign a linkURL to an image (to point towards a custom file), then the Download Button will download the linkURL instead.