I can't get good quality sky_spheres using image_maps.  If I scale them so that they repeat, I get a mirror effect if I look at certain angles (camera { location < -945,287.0 ,  184> look_at <  400, 58.6 ,  0.0> }).  I tried several translations to fix this; all proved futile.

If I use map_type 1, the effect is very blotchy.  I even made a 4x4 tile of the original image in another program and combined that with interpolate 2 in the image_map:

sky_sphere {
 pigment {
  #if (IsTest)
   rgb <0.1, 0.3, 0.8>
  #else
   image_map { png "sky6a.png" map_type 1 interpolate 2 }
   translate <0, -1, -1>
   scale .2
  #end
 }
}

The result was still unsatisfactory.  Try it with a 960x960 png (smaller images show the problem even more) at 1280x1024.  Lowering the render resolution actually improves the sky_sphere, but at an obvious cost.

The map_type 1 option in image_map ignores all other transformations.  The problems I have had with it have led me to openly question this decision.  Simply allowing the frequency keyword here would save a lot of trouble.  'interpolate 2' can only do so much, usually around the edges of the pixel blocks.

There are some who would call this a feature limitation and therefore beyond the scope of beta-test issues.  But if a limit on a feature makes that feature unsuitable for its intended purpose, the 'limit' becomes a 'bug', IMHO.