Override the page properties dialog so that the thumbnail selection is from AEM Assets (/content/dam) instead of a direct file upload.
- Install the attached package sample-thumbnail-picker-1.0.zip to an AEM 6.3 instance.
- Go to http://host:port/crx/de/index.jsp and log in as admin user.
- Browse to this node structure which adds a thumbnail tab (which can be copied to your own custom page properties): /apps/weretail/components/structure/page/cq:dialog/content/items/tabs/items/assetpicker
- Browse to this node, it hides the original thumbnail tab so that only the new one from step 3 is displayed: /apps/weretail/components/structure/page/cq:dialog/content/items/tabs/items/thumbnail