Earlier I used to capture my browser window to get the screenshots of websites for my blog or use this firefox addon to do that. Not only this, after capturing the screenshots I used to resize them using photoshop or MS Paint. This takes a lot of time for so little thing.
Shrinktheweb is online service which will take your blog or website as an input and will give you 3 screenshots of your website or a blog in different sizes which are 320 x 240, 200 x 150 and 120 x 90 pixels.
If this is the first time you are generating thumbnail of your website then it may ask you to wait for less than a minute and will put your request in a queue.