In GIMP, there is a difference between saving and exporting an image.
To save an image, choose File > Save from the menubar, or press Ctrl + S on the keyboard.
In GIMP, saving an image means saving your project as a whole in a way that you can open it again in GIMP, with all of the layers, paths, etc. intact.
By default, the Save Image dialog box will set the extension to XCF. This is GIMP’s native format, and what you’ll want to save your project as if you wish to preserve your work and open it in the program again.
After you’ve chosen your desired location and named your file, click Save.
To export your image, that is to save it as something other than GIMP’s native format (PNG, GIF, JPG, etc.), choose to File > Export from the menubar, or press Ctrl + E on the keyboard.
There are several extensions available in the Export dialog box. You can see a whole list of what’s available by clicking on Select File Type (By Extension) at the bottom of the dialog box. However, most projects you do will use JPG, PNG, or GIF.
After you’ve chosen your desired location and named your file, click Export.
Depending on the extension you chose, a second dialog box may pop up. In most cases, you should simply click Export again. The second dialog box allows you to alter aspects of the file extension, including quality and file size. In most cases, the defaults should be fine.
To save a copy of an image using a different file name or extension, simple choose Save as… orExport as… from the File menu and follow the same instructions.