I used to struggle when I needed to capture screenshots from web application for e.g. user manuals until I found this amazing extension for Chrome called Awesome Screenshot.
With this extension you can capture either the visible part of the page, a selected area of the page or the entire page. It's very easy to use, you just click the capture button in Chrome's extension toolbar and select what you want to capture. It even allows you to add text and primitive shapes to the image. Once you're done editing you can either save the image online, on e.g. Google Drive, add it directly to your clipboard or save it on your computer.
Sounds pretty neat, doesn't it? You can download the extension here: