Hello vinodnr,
We have checked the below auction on your site. It looks like the thumbnail is displayed properly on the auction list and auction detail page.
https://chn.adriaticb.com/auctions/
We have read your second query. You said that the thumbnail images are displayed only if they are uploaded while creating an auction and if you select the existing image from the media library, then it will not be displayed on the front-end.
We have tested the auction product without uploading a new image. The existing image is displayed properly. There is no issue with it. It might be possible that there is some other issue on your site.
Please review the attached video – https://we.tl/t-LwqYCD183P
Currently, if the thumbnail issue is fixed, then it’s fine. If you face this issue again, we suggest that you please record a video of the whole process you have explained and share it with us. So, we will get a clear idea about how you create an auction product.
Thank You