You can choose a thumbnail in the 'Create a New Survey' dialogue, or alternatively, after the survey is created through the 'Edit Survey Info' dialogue when selected from the My Surveys page of survey123.arcgis.com.
Have you tried to update the surveys within the field app after uploading the thumbnail?
If you have done this, how about deleting the survey and redownloading? I have experienced delays in the past, so I'd also be interested to know how long ago you added the thumbnail or created the survey with the thumbnail.
Yes, I have tried both options - multiple times. I added the thumbnail about 45 minutes ago. I suspect that the thumbnail in the Survey123 apps can only be set via Survey123 Connect. Correct?
It appears that we do have an open issue in our backlog outlining this behavior...
If possible, can you try to completely uninstall the Survey123 field app and reinstall it? After doing so, does the correct thumbnail appear?
I have gone ahead and updated the issue with your comments in the meantime.
I am having this issue. Whereby, my form was developed on the Browser and the thumbnail does not show up on the Survey123 app.
iPhone 7 plus
I tried deleting and reinstalling the app without success. I even tried going to AGOL and adding the thumbnail to each feature layer and form found within the surveys folder.