Inserting image and saving doodling separately

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http://www.linuxquestions.org – I have been going over tutorials and man page for gimp, but something is still not clear. I want to draw over an existing image. Fine, create a layer above the existing image. Also fine but how do I put a png or gif file into the original layer? Ok, now I have done my drawing over the original image on the second layer. How do I dispatch the original image so it is not destroyed and does not contain my doodling from the second layer? Can I then save just the second layer with my doodling via save as? If not how? (HowTos)