Article Updated March 2012.
When you take a photograph too close to a tall building you often get converging verticals in your resulting picture. These can really ruin an image by making it look unnatural and distorted, so we are going to cure this problem using Gimp's Perspective tool.
Step 1 - Duplicate Your Layer
Open your image and duplicate the background layer by right-clicking it and selecting Duplicate. This only serves to provide us with a quick way to undo any mistakes we make. Rename the new layer to avoid any confusion that may arise, though this step is mainly necessary in larger projects.
Step 2 - Perspective Tool
Now press Shift and P to activate the Perspective tool. In the left pane there should be some tool options, one of which says Corrective (Backward); this is the box we want checked. In normal mode the Perspective tool will warp the image to fit to the shape you make, in backward mode you create the shape it is currently warped to and the tool will put the image back to a straightened point of view.
Step 3 - Make Adjustment
Drag the tags so that the vertical lines of the quadrilateral are roughly parallel with the sides of the building. Keep the horizontal lines of the selection straight as we do not wish to correct those, and then when you think you have the area about right, press the Return key to apply the transformation. You should be in the right area at the very least, but you will almost certainly have to press Control and Z to undo and try again a few times to get it perfect as this tool requires a lot of trial and error.
Step 4 - Canvas Size
Quite often your image will have had its proportions changed slightly by this procedure. We shall address this next, so first we want to enlarge our canvas. Open Image, Canvas Size and unlink the horizontal and vertical sizes by clicking on the chainlinking the two values. Then, add a good amount of pixels to the top, this is entirely dependent on the size of your image. Don't worry about adding too many we will sort that later.
Step 5 - Transform tool
Press Shift and T to activate the Transform tool, and then drag the central tag on the top horizontal upwards until the proportions of you image look right again. Press the Return key to apply the change.
Step 6 - Rectangle tool
Finally use the Rectangle tool to select your image, and then open Image, Crop to Selection to remove any excess areas and there you have it, your picture looks better than ever.