Using GIMP: Correcting and Touching Up Your Images - GIMP 2.6 for Photographers: Image Editing with Open Source Software - page 95

It is recommended that you use a file format such as XCF, TIFF, or PNG for
your working image. These formats guarantee good image quality. Of course,
if the image contains layers, GIMP will limit your format choice to XCF or PSD.
Compressed files in JPEG or GIF format should be created only as copies
of the original or working image.
The submenus to save images are in the menu bar of the image window.
The File menu offers four Save options: Save, Save As, Save a Copy, and Save
as Template.
Save simply saves your modified image onto itself. The existing version
will be overwritten. After you close the image, you cannot undo this process. I
highly recommend saving your image whenever you modify it to your liking.
This will ensure that you won't lose your work in case of a power outage or
program crash.
Save as : Select this option if your image is new or if you want to save your
image under a different filename and/or in a different file format.
Clicking the Save as menu item opens a new dialog window Save Image .
Here you can enter a name and a filename extension for the image that you
intend to save in the text field labeled Name . The Save in folder drop-down
list displays the location where the last image you saved was stored. Click the
arrow to display the options and select the folder you wish to store the image
in. Click the + sign next to Browse for other folders to enlarge the window and
to show the pane of the file browser, similar to the Open Image dialog. This file
browser will let you find and select the folder you want to save your image in.
Or you can create a new folder by clicking the Create Folder button. Clicking
the + sign next to the Select File Type (By Extension) opens a drop-down list
that lets you select a (different) file type for the image you intend to save.
Always choose a high-quality file format for saving and archiving your
images. You can type in a filename extension or select a file type from the
Select File Type (By Extension) drop-down list in the Save Image dialog. The list
includes the XCF, TIFF, and PNG formats, all with compression. It is a good
idea to create a separate folder for your exercises. (Just click the Create Folder
button in the upper-right portion of the Save Image dialog.)
When you save your image, a new dialog box opens and gives you the
option to export the image in the desired file format, depending on the
format you selected. Click OK to proceed.
Another dialog will now prompt for desired image file attributes. For the
TIFF format, select LZW Compression . This compression method is lossless and
reduces the file size. The remaining format options should be selected only
when you are saving them for your own use. If you want to pass the image
on to other programs, you should be aware that not all programs support the
other file formats or can even open them. Click OK to accept. Now your image
is “in the can”.