Suppose you have an image:
Original image can be found at deviantart.com, copyright olya.
After living with this image for a while, you decide that the pose on the left is the best of the lot and you would like to work with it. First thing you decide is that you would like to crop it down as a stand-alone image. Here's how to do it in Gimp.
Once the image is open in Gimp, right-click it. Select Tools/Transform Tools/Crop & Resize. (By the way, notice that you can also type Shift-C on the keyboard in order to activate the same tool.) When you hover your mouse pointer over the image, the pointer will take on a shape similar to this:
As you begin to drag the initial crop selection, the "Crop & Resize Information" dialog box will pop up. It may pop up right over your image. If it does, just relocate it when you have finished drawing the inital box and before you fine-tune your selection.
I hope the dialog box is self-explanitory; but just in case, the Origin specifies the location of the top left corner of the selection. The Width and Height specify the, well, width and height (respectively) of the selection. You can type your own numbers into this box. If, for example, I wish to crop this image so that it has the same dimentional ratio of a standard computer screen (namely, 4:3, or 1:0.75, or 11/3:1) I might set my prefered height with the mouse, say to 362 pixels, for example. Then I multiply that number by 1.3333 to calculate the proper width: 483. So, I just type that number in for the Width. I press the Tab key (on the keyboard) and note that the shape of the selection on the image has already adjusted to my specification.
Once you have located the selection to my liking, click Crop.
Now, right-click the image. Select File/Save As, and save the image with a new name.