Doesn't legality depends on EULA?
Since there doesn't seem to be one for res files it's most likely legal to decompile, modify, and distribute them in a non commercial derivative works unless they begin to forbid that explicitly.
A few years back we were modifying a closed source product for a client at my dayjob which had only copyright notice to it and no actual license/eula attached. As far as our IP lawyer was concerned it was ok to simply add original owner's copyright to the decompiled sources. The only issue was that the copyirght owner could at any moment ask us not to do that and if refused it might hold grounds in the court (iirc on the basis of potential commercial intent from our side or something like that).