We’ve probably all taken photos where the camera wasn’t perfectly level, resulting in a crooked horizon line or a bent object. It’s easy to fix and straighten a crooked photo with the rotate tool in GIMP.

When you have an image with a crooked horizon, you will need to lose some of the edges of the photo to fix it. The sides of the image will need to be cropped to compensate for the crookedness of the photo due to rotation. You will always need to crop a photo when you rotate it, so it makes sense to rotate and crop in one step with the rotate tool.

Open your photo in GIMP.

With your photo open, move your cursor to the ruler at the top of the document window. Click and drag down to place a guideline on the image. Position the guideline so that it intersects with the horizon in your photo. This doesn't necessarily have to be the actual horizon line as in the practice photo shown here – use anything that you know should be horizontal, such as a roof line or a sidewalk.