That's true of Winamp 2, which was the most popular major version. Its skins were, as you say, basically just bundles of image files that were used to override the built-in assets.
On the other hand, Winamp 3 was a complete rewrite, with a much more flexible theming system based on a custom-designed GUI framework, complete with its own XML-based layout engine and scripting language: https://en.wikipedia.org/wiki/Wasabi_(software)
Unfortunately, it never really caught on, mainly due to the combination of performance problems and a lack of feature parity.
On the other hand, Winamp 3 was a complete rewrite, with a much more flexible theming system based on a custom-designed GUI framework, complete with its own XML-based layout engine and scripting language: https://en.wikipedia.org/wiki/Wasabi_(software)
Unfortunately, it never really caught on, mainly due to the combination of performance problems and a lack of feature parity.