Change default thumbnail

If you want another default thumbnail, you can just enter the complete url of the other image that you would like to use under Default Thumbnail in Output Options

For some reason, I’m still having trouble.

This is the url I am entering:
https://www.alismithtx.com/?attachment_id=311

Placing it in the box that says Default thumbnail:

I have scaled the image to 150 by 150 to match the height and width of my other thumbnails used. Any further advice?

You need to use https://www.alismithtx.com/wp-content/uploads/2014/01/photo-768×1024.jpg

Or any link that goes directly to the image. The link above is to a page and not the image.