I had a lot of trouble searching for the simple task of changing the number of thumbnail images displayed on a page by Plone 3. The answer is to go into the Zope Management Interface (ZMI) and change portal_skins/ATContentTypes/atct_album_view .
Click on the Customize button, then replace the number 12 with the number of thumbnails per page that you want to display in the line:
batch python:Batch(images, 12, int(b_start), orphan=1)">
Reload the page.
Thanks to Mike Takahashi’s post here.