Features of this extension are:
Resize before Taking Image
You can take full resolution page capture or predefine a resolution according to which images will be captured.
Capture visibility area
Here you have got two options either you can capture only portion of webpage visible on your screen or you can capture the whole page.
It resides beside the search bar and can be used by clicking on it. As you take a screenshot using above mentioned options an image preview is shown.
Opening the image brings it up in another window, where you can zoom in, zoom out and save the image. Images saved are only in png format.
Though this extension has limited features and do not stand any chance in front of desktop applications for this purpose, using it will be handy and time saving when you do not have those applications installed on your system.
Download: Webpage Screenshot extension