The online backup service SpiderOak can optionally be used to create online photo galleries. Previous versions of it had problems generating image thumbnails; here’s how I dealt with it.
To my knowledge the software issues provoking the problem have been resolved, so I hope these notes will not be needed by anyone. But for what it’s worth, I did the following.
Per this advice (registration required) from SpiderOak support, I shut down SpiderOak and ran SpiderOak –generate-previews in a terminal. This took quite a while. Then I relaunched SpiderOak. This did not immediately correct the problem.
Per this advice (registration required) from SpiderOak support, I then went to the View tab in SpiderOak and removed all images from SpiderOak that were missing thumbnails. I let SpiderOak upload and sync the changes, and confirmed that the online share no longer had those images. Then I returned to the View tab and added the images again. After again letting SpiderOak upload and sync the changes, thumbnails of images appeared as expected.