Ok, the only thing I can think of, for me not having what you do, is we are not using the same gen_classicart.dll. I'm using version 1.20 that was modified by dro to work better with WACUP. However, your info report says you are using the same version. In my preferences, WACUP plug-ins are listed with a WACUP logo, Winamp plug-ins with a Winamp logo, and 3rd party plug-ins with a question mark. Gen_classicart.dll has a WACUP logo.
If dro sees these posts maybe he can explain why we get different results with gen_classicart.dll. At this point, I don't want to disturb him with a PM about this.
Some of the modern skins I use didn't have standalone album art windows. I added the files I mentioned in my last post to add an album art window that uses the basic frame of the skins I want it in. If I disable my gen_classicart.dll, then I loose the Album Art window in my classic skins and the context menu command, while my modern skins with standalone album art windows are not affected (they still have the window and the context menu command).