I'm using Airport Design Editor to bring my local airport up to real life standards. My question is, though, if I compile the bgl and save it in my addon scenery folder, will it overwrite the default version of the airport or will there be two airports in the same place?

No, because the defaut airports aren't in the Addon Scenery folder! ;)The one you create will simply replace the default airport. It's all covered in the extensive manual that hasn't been read yet... :(

I added a 1120x700 95kb jpeg background image to work with but upon zooming am getting some really bad stutters making difficult to work with. Do changing format or something do good? Also I have noticed the image temporarily move slightly when I click to add objects.

I added a 1120x700 95kb jpeg background image to work with but upon zooming am getting some really bad stutters making difficult to work with. Do changing format or something do good? Also I have noticed the image temporarily move slightly when I click to add objects.

Support for ADE can be found here..... http://www.fsdeveloper.com/forum/forumdisplay.php?f=95