Well, I'll be damned.
When I install an aircraft with no thumbnail it's there but, no picture. It's hard to explain. The aircraft will be there but, it's picture will be of an aircraft that was already there. The new aircraft (without a thumbnail)will take the place of an existing aircraft and use it's thumbnail.It works perfectly once it's selected. Also, every aircraft down the list from the new one is shifted over one spot so that the one you click on isn't the one you get.
I've seen this mentioned here before. Not had the problem myself on two different computers. Perhaps it's a Vista/Win7 thing as I'm still on good old XP. I understand it's caused by the ui_xxxx entries in Aircraft.cfg rather than missing thumbnails. Note the FSX default entries are in quotes like this;
ui_manufacturer=
"Cessna"ui_type=
"C172SP Skyhawk"ui_variation=
" Blue, Gold"
How did you take a shot of your aircraft selection menu? I could show you.
I know this isn't helping at all with DenisH's problem but, it kind of sounds like there might be a problem with my sim :-?
For the screenshots I use the old-fashioned Alt + Print Screen method.