When you have completed your session editing a file you will need to save it. A finished photograph should be saved in JPEG (Joint Photographic Experts Group) format. This has the file extension ".jpg". An incomplete editing project should be saved in The GIMP's own XCF format (with the ".xcf" extension).
When you select "Save as..." the "Save Image" dialogue will appear. As always, the Save Image dialogue requires three pieces of information, the file name, the file type and location. This dialogue will show the original file name, including the ".jpg" extension, which determines the file type, and the folder from which the file was originally loaded.
To rename the image place the cursor in the Name: field and amend it in the normal way. If you are saving a finished photograph the file's name should include the ".jpg" file name extension. If you are part way through an editing session then use the GIMP's own ".xcf" file name extension.
To save the edited image to a new location, first click on the small [+] symbol (or anywhere level with it) beside the text "Browse for other folders". This reveals a file browser element similar to that for the "Open Image" dialogue:
By default, the line under the file browser will say "Select File Type (By Extension)". As suggested above, with this showing, it is enough to add the ".jpg" or ".xcf" extension to the name to determine the file type. However, it is also possible to click the [+] beside this item and manually pick a file type from the list that appears. The next most likely type to choose would be "Photoshop image" (.psd), but only if you plan to pass your half finished project to someone using another image editing program. This file type is the one that other programs are most likely to support and offer compatibility between programs.
If you opt to save the file as a JPEG you will probably be required to "export" the file. The dialogue that appears will indicate why this is necessary - most commonly because you will have introduced features, such as layers, which involve transparency, which are incompatible with the single layer image that a JPEG requires.
Finally, you will be presented with the "Save as JPEG" dialogue. Only the first two controls need concern the average user. "Quality" may be adjusted by the spin wheel or dragging on the slider. Note that clicking on either side of the control adjusts the setting by a factor of ten. Normally, the default level of 85 gives excellent results. Values above 95 do little but increase file size. Values below 75 may be acceptable for web use, where file size may affect download time, but risk the features of file compression showing noticably in areas of similar colour, e.g. sky, car bodywork, plain painted walls,
If the "Show preview in image window" box is checked, a calculation of the file size for that quality level is made as well as the image preview. This allows you to check over the image for the effects of compression, and is especially worth while on images intended for use on the web.
Use for finished images that are ready for display to others.
All computers, DVD players and companies offering printing services can read these files. They offer a good compromise between image quality and small file size, so allowing images to load and display quickly.
The compact file sizes are achieved through compression techniques. Once a file is saved using the JPEG format any loss in the original quality cannot be recovered as some of the image detail is discarded to achieve the smaller file size.
In simple terms, with JPEG compression, an image is examined, block by block, for areas of similar colour. Where they are found the colour value for the whole area is set to an average value. In "busy" areas of the picture smaller blocks are used and no discernable change will be made to the image, but in areas where there is little change in colour from pixel to pixel much larger blocks are averaged. This can lead to distinct recangular blocks becoming apparent in the picture. Watch out for this effect on both clear and overcast skies, on car bodywork, painted walls, or any plain surface, especially at the more severe compressions levels.
Use for incomplete editing projects.
The XCF format is proprietary to The GIMP. It preserves most aspects of the editing process, such as layers, etc, so is ideal if you expect to return to an incomplete project. The cost of this is in file size, which may be massive.
If you are likely to want to swap projects with others with different software, consider using the "Photoshop Image" (.psd) format. This offers similar facilities to the The GIMP's XCF format and is supported by a range of other high quality image editing software.