Where do you get these background :O
HST aka Hubble Space Telescope.
From that gallery, you pick the "Full-Resolution images" option which will, in this case, take you to
Hubblesite.org which has
mega huge pictures for most of the stuff. In this case, source image was a 3857 x 7804 TIFF file.
There are a lot bigger pictures available, such as
Eta Carinae Nebula which has a 29566 x 14321 TIFF file available...
Once I find a suitable source image it's just a matter of editing it in GIMP to produce a spherical texture, which I then use in Blender to produce six-sided skybox texture renders.