It is easy, when taking a picture, to hold the camera not quite perfectly vertical, resulting in a picture where things are tilted at an angle. In GIMP, the way to fix this is to use the Rotate tool. Activate this by clicking its icon in the Toolbox, or by pressing the “R” key capitalized) while inside the image. Make sure the Tool Options are visible, and at the top, make sure for “Affect:” that the left button (“Transform Layer”) is selected. If you then click the mouse inside the image and drag it, you will see a grid appear that rotates as you drag. When the grid looks right, click rotate or press the enter key, and the image will be rotated.

Now as a matter of fact, it isn't so easy to get things right by this method: you often find that things are better but not quite perfect. One solution is to rotate a bit more, but there is a disadvantage to that approach. Each time you rotate an image, because the rotated pixels don't line up precisely with the original pixels, the image inevitably gets blurred a little bit. For a single rotation, the amount of blurring is quite small, but two rotations cause twice as much blurring as one, and there is no reason to blur things more than you have to. A better alternative is to undo the rotation and then do another, adjusting the angle.

Fortunately, GIMP provides another way of doing it that is considerably easier to use: in the Rotate Tool Options, for the Transform Direction you can select "Backward (Corrective)". When you do this, instead of rotating the grid to compensate for the error, you can rotate it to line up with the error. If this seems confusing, try it and you will see that it is quite straightforward.

After you have rotated an image, there will be unpleasant triangular "holes" at the corners. One way to fix them is to create a background that fills the holes with some unobtrusive or neutral color, but usually a better solution is to crop the image. The greater the rotation, the more cropping is required, so it is best to get the camera aligned as well as possible when you take the picture in the first place.

When you take a picture with a digital camera, you have some control over what gets included in the image but often not as much as you would like: the result is images that could benefit from trimming. Beyond this, it is often possible to enhance the impact of an image by trimming it so that the most important elements are placed at key points. A rule of thumb, not always to be followed but good to keep in mind, is the “rule of thirds”, which says that maximum impact is obtained by placing the center of interest one-third of the way across the image, both widthwise and heightwise.

To crop an image, activate the Crop tool in the Toolbox, or by pressing the “C” key (capitalized) while inside the image. With the tool active, clicking and dragging in the image will sweep out a crop rectangle. It will also pop up a dialog that allows you to adjust the dimensions of the crop region if they aren't quite right. When everything is perfect, hit the Crop button in the dialog.