Take a screenshot from a web pages or your own website is something usual, generally that is by pressing the Print Screen button on your keyboard keys and then paste it into image editor like Photoshop.
By using the Print Screen button on your keyboard is enough, but if you want more then you must look for software that is able to meet your needs. Suppose you want to take pictures for the entire page, specify the image format and others.
For the needs as I already explained above, I use Pearl Crescent Page Saver. Pearl Crescent Page Saver is an extension for Mozilla Firefox that lets you capture images of web pages, including Flash content. System requirements: Firefox 3.5 and 3.6 on Windows XP, Mac OS X, and Linux x86. Page Saver work on all platforms where Firefox is installed.