The default file format in GIMP is XCF, but this is only used for editing images within GIMP. Once you've finished working on your image, you need to convert it to a suitable format, depending on the type of image and how it is to be used.
One such option is to save to a JPEG, which is a popular format for saving photo type images. One of the great things about JPEGs is the ability to use compression to reduce the file size, though it should always be noted that as compression is increased, the quality of the image is reduced, and this can be very significant if high levels of compression are applied.
1. Save the Image
The first step is to go to the File menu and click on either the Save as or Save a copy commands. The Save a copy command will keep the XCF file open within GIMP, while saving a version of the image as JPEG. In the dialog that opens, you click on Select File Type to open the list of file types and then scroll down and click on JPEG image before clicking the Save button.
2. Export File Dialog
Assuming that your original file has more than one layer or contains transparency, the Export File dialog will open. You can usually just click the Export button and leave any optional settings set to the default.
3. Save as JPEG Dialog
The Quality slider defaults to 85, but you can adjust this up or down to reduce or increase compression. Clicking on the Show preview in image window checkbox will display the size of the JPEG using the current Quality settings – it may take a few moments for this figure to update after adjusting the slider. This also opens a preview of the image with the compression applied so that you can ensure that the image quality is acceptable before saving. You may have to manually select the preview image in order to see the effects.
4. Advanced Options
You can expand the Advanced Options settings, but most users will be best off leaving these settings as they are. However, if the JPEG is large and intended for the web, the Progressive checkbox will make the JPEG seem to display more quickly, because it comes in at a lower quality setting initially. This is also known as interlacing, though it is less often used now with faster internet speeds.
It is quite straightforward to save images as JPEGs in GIMP and the application offers a range of more advanced settings for those who like a greater degree of control when saving their images.