Earlier I posted the HOW TO : Add Random Post Widget for Blogger/Blogspot and if you want a gadget which shows thumbnails, as showing in the image given below, from your blog then here is the solution for you.
2. Select the preferences which you want for your Random Post widget
3. Save the gadget and move it to where you want it to be appear on your blog.
4. All done. Now check your blog for the Random Post widget :)