I just took a screen shot and I got a *png as a result, in my ~/PIctures folder. No problems there.
I fired GIMP, cut a piece out of the picture, opened a new file, pasted and saved the new image file. However, GIMP only gave me the choice of saving it as an .xcf. Why? When I used GIMP on other distros before I never had that problem. What can I do to change this?