Import your nebula image into Gimp, export as .dds file with the dds exporter plugin, save as DXT1, put the new dds file in data/textures/sky (create folders in Skyrim data folder if not there) and rename as skyrimgalaxy.dds. I think the star twinkling effect is caused by having no minimaps enabled, but I'm not sure. Hope this helps. edit: it's better to save it as DXT5 actually. I checked the galaxy file and they had transparency enabled, so when making your dds file, remember to make the area around the nebula transparent.