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.