We have covered quite a few desktop application for creating thumbnail previews in Windows. Thumboo is an online application for creating different size thumbnails of websites. All you need is to enter the website URL and it creates four different screenshots of the website. The site also provides code for the thumbnails, which can be copied and pasted on your site where you want the thumbnail to appear.
You can enter only website URLs using this service and it cannot resize online images.
The service provides a PHP code for each thumbnail which you can copy paste on your website to show the images.