You don't even need to fly to grab a screenshot for a thumbnail.
Load the plane in the spinner, press "PrntScrn" to save to the clipboard, load up your paint package of choice, I use Irfanview. And select "paste". Crop the image to just have the aircraft & save as thumbnail.jpg, drop the thumbnail into the correct folder. Then when you click on "change" on the current aircraft section of the menu it will now display the thumbnail for the new aircraft, repeat until all the missing thumbnails are in place. This also helps if it's difficult to work out which aircraft belongs to which folder. If it's wrong you immediately see the mistake and move it.

eg

Ps. Failing that what I have also done in the past is get the ID of the aircraft eg ZD180 type it into Google and find either a real photo of the aircraft or another screenshot
from elsewhere. <---- lazy method.
Load the plane in the spinner, press "PrntScrn" to save to the clipboard, load up your paint package of choice, I use Irfanview. And select "paste". Crop the image to just have the aircraft & save as thumbnail.jpg, drop the thumbnail into the correct folder. Then when you click on "change" on the current aircraft section of the menu it will now display the thumbnail for the new aircraft, repeat until all the missing thumbnails are in place. This also helps if it's difficult to work out which aircraft belongs to which folder. If it's wrong you immediately see the mistake and move it.
eg

Ps. Failing that what I have also done in the past is get the ID of the aircraft eg ZD180 type it into Google and find either a real photo of the aircraft or another screenshot
from elsewhere. <---- lazy method.