Problem: How to create thumbnail preview images for a skin in DNN 7.x(DotnetNuke) ?
You have to create a jpg with the same name as the HTML or ASCX file
For Example: Your Home page had name called index.aspx
Take a screenshot of your Home page and save the image in JPEG format with index as name and place the file in your skin folder.
To see the thumbnail is applied or not Navigate to Admin -> Site Wizard from control bar.
Hope this is clear and solved your problem.Let me know if you have any questions . TO know more about me you can visit my portfolio: www.nithinchandrasr.com