We will need a bit more info as to what you are trying to do. Have you edited the photo and want to email as it is now? Or wanting to reduce it in size? Or ...?
A general rule: In Gimp SAVE as a .xcf file. Then EXPORT as a .jpg (or other format).
As Blighty says. File>Save is only for the XCF format that saves all you work (added layers, selections, paths...). To produce "display" formats such as JPEG/GIF/PNG use File>Export. When you leave Gimp or try to close an image that you didn't save as XCF, Gimp will ask you to confirm. This is a bit painful at teh beginning, but 1) you can get the Ctr-lD reflex. and 2) you will be glad you have this when you start doing complex things with Gimp.