This ship is the USS New Jersey (BB-16), painted with camouflage in 1918. It's one of the subjects of a Wikipedia content drive called Operation Majestic Titan, which is working to provide good articles about every battleship ever built. An administrator who works with that project, The ed17, thought this photograph might have the quality to become a featured picture.
Ed asked me for help after the first reviewer noticed how tilted the horizon is. He didn't think the defenseimagery.mil image was high enough resolution to merit a full restoration--he just couldn't figure a good way to correct this tilt. Ed's dilemma was that he had no leeway. The photograph already cuts off part of the ship's superstructure. He didn't want to lose any more information.
If the solution looks confusing at first, smile and think how much it will impress your friends.
The first thing to do is straighten that horizon: this image needed two degrees counterclockwise rotation. That's why the framing looks tilted within a black box. The next step is the crop. The dotted lines on this screen shot show where the crop was about to happen.
Ed understood the basic mechanics of editing but he wasn't sure how to apply them to this situation. This image had enough room to crop left and right and a little too much foreground, but that superstructure left no good choice at the top of the frame.
Obviously we aren't going to propose a featured picture candidate with black triangles in the sky, but that's just sky. We can patch empty sky.
These edits used the clone stamp and then the healing brush to blend a natural effect. Altogether it took less than five minutes to perform the rotation, crop, and the repair on the sky.GIMP can do it. The Wikipedia article about GIMP includes external links for download and tutorial.
And if you're more likely to edit birthday party photos than World War I history, a good habit to get into is to zoom out a little while shooting. The editing is much easier when all four sides of an image have enough room to rotate and crop.