Visitors selecting a theme

Isn’t the theme-switcher working on 1.5?
https://dev.wp-plugins.org/wiki/ThemeSwitcher

Thanks for the plugin link! However, are there any ways to replace the theme name with an image instead? Access to another theme is only possible by clicking on the links provided by the tag <?php get_theme_switcher() ?>, so i can’t type like https://www.____.com/?wptheme=___ directly into the browser?

Yes, you can. Just watch the address in the toolbar of your browser when hovering over the links provided by the tag.