Hi,
I think I know what you mean.
When you create a graphic and save it, gimp saves it with its native file format (default extension) .xcf. All graphics programs save your work in their own native format by default so they can be easily edited later. To use your image in websites etc. you need to also save it with a .jpg or .gif extension.
To do this - load your image/work in gimp, go to 'File'>'Save as'.
Give it a new name or leave the name as it is. Expand the 'select file type' menu
This is the IMPORTANT PART: Scroll down the menu and select the .jpg or .jpeg option (or .gif if you wish).
Click 'save'.
You should now have another image file with the .jpg extension which you can use mostly anywhere.
Hope this helps,
-Uni