Is there a lightweight tool to crop images quickly?

Gthumb is a nice image viewing/editing tool with simple editing tools like cropping.

Instructions for cropping in gThumb 3.2.8

1. Open your image in gThumb

2. Open the Edit sidebar by pressing e or clicking on the easel in the top-right corner of the window

3. In the sidebar click on Crop and then crop the image as you desire

4. Once finished press Enter or Crop. Then, in the Edit sidebar press Save to overwrite the original file or Save As to save to a new image file.

On the command line, the tool to manipulate bitmap images is imagemagick or graphicsmagick (GM is a split of the IM project, and more actively developed). This is a good option if you often use the same parameters.

convert raw.jpg -crop 800x460+100+20 cropped.jpg     # ImageMagick
gm convert raw.jgp -crop 800x460+100+20 cropped.jpg  # GraphicsMagick

For ad hoc use where you need to see each image, you can use display (also from the ImageMagick suite) or gm display (GraphicsMagick) or Shotwell or Pinta or many other image viewers with light editing capabilities.

You can crop and export pretty quickly with Shotwell, it's pre-installed. Failing that try Pinta, it's in the Software Center.