Hi. I think someone say is just need to remake the cubemap texture with the cubemap tool texture but here I talk if we haven't the raw file (but I remember, I think cubemap can remake the raw file).
Software require : Irfanview (free)
Screenshots are French, so I try to translate.
First, you need to open the software, after this, click on "File\Convert/Rename on Batch mode" :
You arrive to this windows :
Next, go to the planet textures folder (addons/MyAddons/textures/planets/MyPlanet), select the "Output format" (for me, is PNG), check "Advanced options" and open the "Advanced options" :
Check "RESIZE" and put for weight and height 256 and select the units "pixels".
Also in "DIVERS", check "Erase files exists" (if you want to make a backup, made now) and click "OK".
After, check "Include child-folders (for 'Add all' no rappel)", set the "Output folder" "$D" (ie the same folder of the original files) and click on "Add all". You have a similar window like me (but depends of your translation) :
Check files selected and you can delete a file with select a file and just click to "Delete".
For final, click on "Start" and when is finish, you have this window :
You can close the software and launch Space Engine and enjoy.