The default file format in GIMP is XCF, but this is only used for editing images in GIMP. When you have finished editing your image, you will need to convert it to a suitable standard format for use elsewhere. For example, you can export a file as a JPEG in GIMP.
Save or Export an Image as JPEG, PNG, GIF, etc. – GIMP 2.8 for Beginners
To save an image in JPEG format using GIMP:
Select File > Export As.
Use the Export As box to assign a name and location to the image.
