For current Mac users who reach Kathryn’s “open each model individually” problem: I wrote Mac SKP Icon Repair & Keeper 0.1.5, a free extension with whole-folder repair. This is a separate tool, not an update to John’s Add Icons plugin discussed above.
Choose Extensions > Repair a folder of SketchUp Finder icons… and select the actual model-library folder. Normal subfolders are included without opening every model. It uses the preview already embedded in each .skp as its Finder icon, leaves model contents unchanged and skips files without a preview. It also restores the current model’s icon after ordinary saves. It does not fix crashes, recover unsaved work, or repair the Quick Look service.
Download Mac SKP Icon Repair from SketchUcation PluginStore. Source, instructions and an alternative installer are in SamTSham’s sketchup-icon-repair repository on GitHub; my announcement in Extensions & Applications Discussions links to both.
Tested on Apple silicon with SketchUp 2021–2026, not the earlier versions in this discussion. Intel hardware testing is unconfirmed. Folder repair requires Python 3 at /usr/bin/python3.