Post-Processing Workflow in GIMP
Transformations (rotate and perspective)
Not always can we get our shots perfectly level or have tilt-shift lenses, so we will use transformation tools to set things straight. These tools can be found in the menu (Tools -> Transform tools) or in the toolbox.
Rotate
If the whole image must be rotated by straight angle (90° or 180°), use the Image -> Transform menu.
Rotation by arbitrary angle is done with the command Tools -> Transform tools -> Rotate. This image needs rotation badly:
Since we are using rotation not as a creative effect, but to correct a mistake, select 'Corrective' direction in tool options. Click on the image. Now rotate the grid until it lines up with the original image. For example, so that horizontal grid lines are parallel to the horizon and vertical grid lines are parallel to the vertical lines:
To apply the transformation, press the 'Rotate' button:
Oops! Some parts of the image are now transparent. They should be cropped off (see cropping below).
Perspective
Most pictures have somewhat skewed perspectives, and usually it's not a problem (and sometimes it can be a feature), but in this case the image can be improved:
First, select the perspective tool (Tools -> Transform -> Perspective), select corrective mode in tool options (again, we are correcting defects), then click somewhere on the image. To change the perspective, take image by the corners and move them. For instance, in this image, place the grid lines along the sides of the building:
And press the 'Transform' button:
By the way, perspective correction takes care of rotation as well, so you can skip manual rotation if you will correct the perspective.