If you change the cloud plane images from the default and then try to reset your sky using the reset sky option from the Sky&Fog palette, the cloud plane image will not revert to default launch cloud plane image. If you want this ability, you will need to add the default sky settings to your preset sky library and apply that preset when you wish to reset the sky to the default.
It is possible to save the sky as a sky preset in the Sky Preset Library. It is also possible to export one or more sky presets onto the hard drive as a single file in Bryce Sky Preset format (.bsk). This exported preset can be imported into Bryce. Be aware that if the sky preset uses features not available in an earlier version of Bryce (such as HDRI which is not available in Bryce 5.5 or earlier), the sky preset make not work in that earlier version of Bryce.