I saw how to do this a couple years ago, but can't remember where and can't find it! How do I change a airplanes thumbnail? Several of my airplanes thumbs don't show up
Just make a screenshot of your plane (pressing V by default or pressing "print" key and pasting in paint) then save it into the texture subfolder of the airplane dir. with the name thumbnail.jpg
If the airplane folder has more than a texture subfolder you have to save a thumbnail.jpg file for each of the subfolders
P.S. I found some airplanes with the thumb file saved with the wrong name i.e. tumbnail.jpg or thumbnails.jpg; in this case just rename it correctly as thumbnail.jpg.