This is now working as intended in the current version of ArcGIS Online (AGOL).
解决办法
1. In the build mode of the web app, click on any of the images in the Table of Contents and it expands the view of the selected image.2. In the expanded view of the image/feature, the contents of the fields and the thumbnail image can be viewed.3. Now click near the thumbnail image, give a space and save the web app. Now open it for viewing the web app outside the build mode.4. With the space near the thumbnail, it is visible in the web app outside the build mode as well.