The picture is Loch Laggan, it's certainly a bonnie part of the world. I use quite a few of my photos for desktops, they have memories as well as being pretty (sometimes).
As for posting desktops just open it up and hit print screen to copy it to the clipboard then paste it into an image program (MS Paint is sufficent for those using windows). A nicer thread look can be achieved by uploading a thumbnail too then having it as the link to the main image.
Here's one of the view from the top of Beinn a' Chlachair

Look behind you and see an ever decreasing number of ghosts. Currently about 15.