Before Windows 7, you needed multiple tools and keystrokes to get the screenshot of your WordPress theme. On Windows 7, it is very easy with the snipping tool.
Open the browser to a post or page on your website that you want to appear on your screenshot.
Click the start menu on Windows 7. The snipping tool is usually at the top. If you cannot find, just type snipping tool in the search box. Start snipping tool.
Make sure you have rectangular snip selected in the new drop down on the snipping tool.
Now move the mouse cursor to the browser window with your themed blog post on it and select the area you want it on the screenshot. Once you complete the selection, snipping tool will capture that area and show it on its own window. Save the image as either screenshot.jpg or screenshot.png. Note that snipping tool adds extensions in upper case. Convert it to lower case. Unix/Linux is case sensitive and may not serve up the file.
Place this file in the theme folder (directory).
That’s all to it.