'ello Elizabeth, you can open many types of files (tif, jpg…) in GIMP. If you edit them adding layers and other Gimp-specific tools and you want to save the image including those layers, you must save that file as a xcf, an internal Gimp format. Xcfs can’t always (nearly never?) be shown in image viewers like gThumb.
If you want to save your edited file in ‘regular’ formats like jpg or tiff, you must export them and choose PNG or JPEG or whatever.
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