SharePoint Online Picture Library doesn't display thumbnail previews
You have a picture library that's using the classic experience for lists and libraries in SharePoint Online. You add a large image to the Picture Library. However, when you use the Thumbnails view for the library, the thumbnail image isn’t displayed. Additionally, when you click the image, the Preview field displays a broken image-link icon and the text "Picture."
To work around this issue, reduce the image to less than 50 megapixels, or use the new SharePoint Online Document Library view.
Although the thumbnail preview for the image is affected by this issue, you can open the file as expected by clicking the file name.
This issue occurs when the size of an image exceeds 50 megapixels in a picture library that uses the classic view.
For more information about the new look and navigation experience for document libraries, go to Switch the default for document libraries from new or classic.
For more information about how to work with photos in a picture library, go to Work with photos in a picture library.
Still need help? Go to Microsoft Community.