Using GIMP: Correcting and Touching Up Your Images - GIMP 2.6 for Photographers: Image Editing with Open Source Software - page 89

The following representation shows how resolution, image size, and
quality interrelate:

The original as acquired from
the camera.
1200 px x 1600 px =
16.667 in x 22.222 in at 72 ppi

Reversible The image as set

to print size:
1200 px x 1600
px =
5.25 in x 7.00 in at
228.571ppi

Not reversible The image as

recalculated for
the Internet:
378 px x 504 px =
5.25 in x 7.00 in at
72 ppi

Without recalculation
(print size):
Number of pixels remains the same,
quality (image information) and
file size remain the same.

Number of pixels are reduced,
quality (image information) and
file size are reduced.

If you choose to enlarge an image, you must reduce the resolution by
the factor by which you want to enlarge it. In this case, the entire number of
pixels remains the same. The resolution and thus the print quality is reduced.
To enlarge the image choose Image > Print Size . When printing, you should
consider the fact that a resolution of less than 150 ppi will often produce poor
results, even on a modern ink-jet printer. Generally, a resolution of 220 ppi is
considered the bottom line for a good print.
There is an option to artificially enlarge an image using interpolation
to increase both the size and resolution. This process calculates new image
dots and adds them to the image. But if you enlarge an image beyond a
certain size, it will usually become spongy and blurred. The existing image
information is simply enlarged, and no additional details can be added later
through calculation. Existing faults in the image are enlarged too, such as
edges that appear from sharpening the image in GIMP. Nevertheless, there
are qualitative differences depending on the interpolation method. Tests have
shown that results with enlargements of a factor of 16x were still satisfactory.
My own experience has shown that image editing programs such as GIMP can
produce satisfactory results with a factor of 8x to 10x. The source of the image
affects the quality. If it has a lot of contrast, sharpness with lots of detail, you
can enlarge it more than if it were faint and blurred. You can enlarge as well as
reduce images by selecting Image > Scale Image .