You can find a lot of high-quality, free images at Pixabay. Try searching there, you're surely going to find something that catches your fancy. Those are all under creative commons license, so you don't even need to attribute anyone (unless you want to do).
Personally I prefer the Hammerhead to the Leviathan. It carries less troops, sure, but I always feel I have to use the Leviathan to its capacity and that just makes the missions take so very long. Unless you bring a lot of aquanauts instead of SWS.
In X-Com: Terror from the Deep, the Leviathan is the ultimate vehicle, basically the Avenger from the first game. It's also the name of a tank in the W40K universe, but to anyone who's not played Epic that seems a less likely reference to get :) Good old TftD... I wonder when Firaxis is going to announce they're working on it.
Ah ok.. that was the one Xcom I played the least. Played the first and the third (or forth, depending on how you measure. There was a flight sim in between I think) a lot more than the second, which IIRC, was TftD.
The "flight sim" was Interceptor. Never tried it myself. I'm unsure if it was 3rd or not, though Apocalypse is considered the third of the "main series" (Enemy Unknown, Terror from the Deep, Apocalypse). Wonder if Firaxis will redo Apocalypse as well. They should hurry up and announce TftD already, they've been hinting at it long enough.
Yes, that's the one. Never did try it myself, Interceptor, as I didn't fancy the concept. I actually liked Apocalypse a lot, but I recall some fans didn't since it was a step away from the classic concept. The forth one, don't mention it's name, was almost as horrid as HoMM4! I get creepy chills just thinking about how bad that wretched abomination was, poor thing.