I was looking for this.

Solution is perfectly elegant, I should say I am using ShowKase Pro with Juicebox Pro.

Create your gallery in Lightroom with the largest size images that you want.

Using the admin on the online gallery, got to the 'site' tab, click on 'customise viewers' tab.

Click on 'Edit' Juicebox 5.5.1, scroll down to:
Pro Viewer Options
Multi-Size Images (Pro)

and put in your sizes there, when you upload from Lightroom and import the gallery in Showkase, the medium and small images will be created (and your large images processed) and the images directories populated accordingly. It's neat.

nb: use the customisation to override any import settings, tried and tested

Hope that helps anyone looking for a solution, I found this minutes before I was going to post the same question here, as it was quite confounding and took too long to work out.

Might be helpful if this was noted in the Lightroom plugin instructions on the website Steve.

Hi Steve,

Thanks as always for your prompt reply to my very rare requests for help.

I realised that I am using Lightroom Classic (the CC name confused me, sorry)

I have done all of the things you suggested, I tried to post a screen shot but failed, I will try again. Sorry I did not make myself clear.

Users/username/Library/Application Support/Adobe/Lightroom/ There is a Web Gallery directory in this location. It appears that lightroom shares the same directory as Lightroom. I have followed your instructions to the letter,  Double clicking in the plugin folder and indeed I already put the juicebox 'plugin' into the Web Galleries directory by hand. Double clicking leads to the juicebox 'webmodule' being deleted.

[*]SOLVED[/*] With Lightroom not open/running, with the juicebox plugin in the plugins folder, and also put by hand in the Web Galleries directory (Users/username/Library/Application Support/Adobe/Lightroom/Web Galleries'), on launching LightroomCC the juicebox web module is available. (to find the hidden library directory see this link https://nektony.com/blog/how-to-get-to-library-on-mac.)

Please note I am using the most recent (to date) software and OS.

I suggest that the instructions need to be slightly clarified, as double clicking on the plugin does not achieve the same results as previously expected, also NB. If you put the Juicebox webmodule/plugin in the Web Gallery folder manually and do the double click procedure, the juicebox webmodule/plugin gets deleted from the Library Web Galleries directory.

I have not used it yet, but to reiterate: Putting the Juicebox plugin manually in the existing "users/username/Library/Application Support/Adobe/Lightroom/Web Galleries" and launching Lightroom Classic makes it available.

As Steve says the web module is not found in the plugins when using Lightroom Classic, go to the top right menu click web.

I hope that helps anyone else with a similar issue. :-)

Looking forward to trying this out, l love Showkase and Juicebox, this will be the icing on the cake.

I have worked it out - no reply needed. Please block the phisher though, thanks.