This is common problem which most users don't see or in many cases didn't notice. (Sorry for my english)
As posted before some sliders do most things wrong when they are at full right.
Go to the general graphic settings on SETTINGS -DISPLAY; GRAPHICS and check your GLOBAL TEXTURE QUALITY.
When I changed it on MEDIUM I have got same result as you have on your picture, correct it on maximum and check result.
Basicaly this setting is affecting all global textures like walls on default generic houses, trees and so on. It affect even outside plane surfaces. If you are using only addons aircraft you will not see any difference when changing this, maybe that's why you didn't see any reason to move this slider higher to see changes. But this setting is affecting all build-in aircrafts, boats, cars, generaly all build-in objects delivered with your FSX package and F-18 is one of them...
Hope it helped, because generaly whole simulation after this setting on HIGH or VERY HIGH will look much much better but don't overshoot the resolution on your graphic card otherwise you will have low FPS...
I have 8800GTS 320MB