Is your image crooked? Not a problem, there is a quick fix for that. Switch to Measure tool - under the droplet (Shift-I) and draw the line along your lines in the picture, which should be vertical or horizontal. It could be the horizon line or a wall of a building - simply mark the line that should be straight. And the go to the main menu - Image, Rotate Canvas, Arbitratry. The angle will be already there. Press OK and here it is - your image is straight, you just have to crop off the unwanted areas.