There is no official preview. It would take too long to go and take screenshots of every skin then upload them into a database and link the screenshots to the file name, etc... At least that's the gist of it from comments Skuzzy made once upon a time.
There is an unofficial one. Look for the Aces High Skins Catalog in my signature. It's outdated. It doesn't have new planes, and the skins have been rotated once or twice since then so some of those may not be in game anymore, and others may have taken their place.
Note the download links go directly to the HTC website. So bf109f_13 or p51d_10 (random examples) may not be the same skins as shown in the screenshots.
Compare the names of the skins to the names given on HTC's downloads page. If they match, there is a good chance the screenshot is valid. If not, don't trust it.
EDIT: Pulled it from my sig:
http://www.netaces.org/skins/skins.htm
