Nice one getting the Q400 shot on to jetphotos. Just to clarify, you do not need to add categories to most photos uploaded, unless it is a special livery, etc.
As to the rejected photo, at first glance it looks level enough but I'm in uni without photoshop so I can't be sure. The large white tank on the lefthand side in the background looks level, whilst the tank on the right side seems to lean slightly to the right by a tiny bit.
To be honest I would appeal the rejection and mention that the photo appears level. You've nothing to lose by appealing and the chances are the appeal screener will accept it.