not at this point, unless you would alter the theme-switcher and themes
What you could do is:
1) Make a new setting in the theme-files style.css called theme-image (or what ever)
2) Where the theme-switcher reads out the style-sheets (as I think it does) make a new entry 🙂
Make note that to my knowledge a PULLDOWN menu can NOT handle images !!!!! So this would ONLY work with the normal setting
It is possible.
But it isn’t as fancy as a plugin or something.
Just insert an image and make a link of it.
the URL is at mine http://rjfalcon.com/index.php?wptheme=Gemini+Green
the Gemini+Green part you can change to your theme..
and of course use your own site in stead of rjfalcon.com.
It works with me.
Don’t know if you need the wp-theme-switcher plugin. I have it activated and it works.
Hope it works for you..