2. Rotating and cropping
First we'll fix the tilted camera. You do this by selecting the Rotate tool.
When this tool is selected, clicking in the image will superimpose a grid on it that you can rotate by dragging it around. Or, you can select the rotation angle with the slider or text area in that "Rotation Information" box. Clicking on the "Rotate" button will rotate the picture according to the grid.
This tool has a bunch of options, most of which you can ignore. The really important and clever one is "Transform Direction".
When the Transform Direction is "Forward (Traditional)", the grid's orientation represents what the picture will be rotated to from its original upright position. If the picture is already upright and you want it to look tilted, that's the way to do it.
The "Backward (Corrective)" direction is what you use to fix a picture that you took with a tilted camera. You orient the grid so it shows how you think the picture is tilted. I've selected that, and oriented the grid so that it coincides with the tilt of the bookshelf behind Niobe's head. Clicking on Rotate will then un-tilt it.
If you get the angle wrong you can always undo the rotation with Control-Z and try again. You can undo almost anything you do by hitting Control-Z (there are some things that can't be undone, but when you do them, they have big warning dialogs telling you that). Undo is your friend! GIMP remembers the last several things you did, so you can undo several operations in succession. Anything you undo, you can also redo with Control-Y. It takes most of the anxiety out of messing with pictures.
You can also go back to the last version of the picture that you saved to disk by using File->Revert under the picture's File menu.
The picture looks upright now, but it has a strange tilted border, and it could use some cutting down anyway to cut out the uninteresting stuff around the edges. We can do that with the Crop tool, which is the thing in the Toolbox that looks like a knife:
Once you've selected this tool, you can click in the picture and drag out a cropping rectangle. You don't have to get it perfect, because it doesn't cut down the picture immediately. Instead it shows a couple of L-shaped lines that define the rectangle. You can drag them around by their corners, and when you've got them just how you want them, clicking on the Crop button in the dialog will actually crop the picture.
Cropping a picture so that it looks good takes some artistry. It isn't necessarily best for the subject to be at dead center. There is a piece of photographic lore called the "Rule of Thirds" that says that it's good to put the focus of attention about one-third or two-thirds of the way across the picture, either in the vertical or the horizontal direction or both. I don't follow that absolutely mechanically, but here I've put Niobe's eyes about a third of the way in from the left.
The view window in GIMP shows a scaled version of your picture. If the picture came from a digital camera, the view is probably scaled down from its original size. When the picture changes in size, either by cropping or rescaling it (which I'll cover later), or if you just want to look at a piece of it in greater detail, you'll probably want to change the view magnification. The + and - keys increase and decrease the magnification of the view window. (It's annoying, but just pressing = does not do anything; you have to hold down the shift key to make it +.) These keys do not do anything to the picture you are editing; they just vary your view of it within GIMP.
When the picture is magnified so that it's bigger than the window, you can use the window sliders to move your view around. Another useful key combination is Control-E ("Shrink Wrap"), which resizes the window to fit the picture, to the extent that this is possible. If you've scaled down the view so that there are gray borders all around the picture, Control-E will shrink down the window to eliminate them.
Next: Adjusting color and brightness