Gimp is an excellent tool for precisely cropping an image. Once Gimp is loaded, you an open an image by either using File → Open (Ctrl-o), or using invoking Gimp from the command line passing in the path of the file. Because Gimp takes a long time to load, it is not worth having multiple instances of it open; rather, have one instance open, and open the files you wish to edit in this one instance.
Once you have opened the image in Gimp, select the cropping tool:
Draw a rectangle anywhere on the screen. In the Tool Options, in the `Size` textboxes, set the aspect ratio, for example, 16 by 9.Then tick `Fixed` and select `Aspect Ratio` drop the drop down:
Because the aspect ratio is fixed, if you enter a new size value in the Size textbox and tab out of it, the other size value will change according to the ratio.
Drag the rectangle around to capture your desired region. Click on Image → "Crop to Selection". Save this new image.