I can't see any thumbnails!

The most common culprit for missing thumbnail images is Jetpack's Photon extension. If you have Jetpack installed and Photon enabled, you will need to disable it. Unfortunately Sunshine Photo Cart and Photon simply do not work together at the moment.

Check whether the photos are still processing

If you have delayed image processing turned on, photos are not shown until their thumbnails have been built. Until then you see a plain placeholder, and in the gallery admin those photos are labelled Processing.

That is normal for the first minute or two after a big upload. If the label is still there much later, or says Processing failed, the background queue has stopped. Check Sunshine Photo Cart > System Information, which tells you how many photos are waiting and when the queue next runs. The usual cause is WP-Cron no longer running on the site.