You have to use very specific encodings and thumbnail sizes for the image to display. We already released a profile for the Ender 3 V2 that should work without any changes. Go to Help > Configuration Assistant, type in "Ender 3 V2 Neo" and then download that profile. We have tested this with our own V2 Neo and it displays the thumbnails correctly.
Many printers require specific thumbnail resolutions, so the settings that work for one machine may not work with another.
I believe the issue in this thread is solved, but if you are still having trouble with your machine first try going to Help > Configuration Assistant within S3D to see if we already created a profile with thumbnail support. If you're still having trouble, please make a new thread.