markodemarko, I don't know much English, but I'll give it a try. ) To open the icons I downloaded, you can use L2Editor.
Try also to use the files that Monokini2 downloaded. He has in his archive the source icon files (in the right DDS format), as well as a script that can pack them all (including your custom icons, which you will need to add to the folders with the sources and the script) in the package icon.u. Or you can also open them via L2Editor, and transfer manually icons from your package to this one via rename function.