Using GIMP: Correcting and Touching Up Your Images - GIMP 2.6 for Photographers: Image Editing with Open Source Software - page 131

Click Scale to accept your changes. The image will become smaller in the
image window, but this time, it's not a zoom effect. The image has actually
become smaller; the pixel count has been reduced. Because some image data
is lost in the reduction process, you should always use a copy of the original
image. If you forget to save a copy first, you can discard the changes when you
close the original.
To get a better view of the changes you will be trying out next, use the
Zoom function to enlarge the view to a zoom factor of 100%.
Choose Filters > Enhance > Sharpen to sharpen the contours of your
image. This filter has little or no effect on high-resolution images. Once you
have reduced the resolution of an image, however, the Sharpen filter can
greatly improve the edge sharpness and clearness of the reduced contours.
To save an image in a compressed format (JPEG, PNG), start in the usual
way: Choose File > Save as to open the Save Image dialog. Select the folder
where you want to store the image. Enter a filename for the image and select
a file format; in this case, you'll choose a compressed file format, such as JPEG
or PNG .
Figure 2.57
The Export File
dialog for saving
an image in JPEG
format