Ok, perhaps we already lost some of you but let's take it a deep further, shall we?
I'll take the same website where both online apps and CLI failed, http://pomodrone.com. It's an easy one since it's only a few different screens and like @luxlogica said for more complicated ones you'll have first to inject some code to do it properly.
Open the page in Firefox, take one a screenshot of each different view with the CLI by typing
screenshot pomodrone02.png and so on.
You'll now have 5 different pngs:
Ok, we need to merge them. You can use Photoshop or any other app but I'll show you an easier way with the console (Terminal or Cmd.exe) by using ImageMagick.
You'll learn more about it here.
For that you must install it with the great help of Homebrew. It's THE package installer for OS X. You also have Chocolatey for Win but I'll stick to mac for now, PM me if you're interested to know how to do it on Windows.
HomeBrew is just a lifesaver. It can install packages for you without asking for root access and can be very useful, from installing your dev environment to completely install your new computer for a fresh start (w/ Homebrew Cask and dotfiles).
01 - Install Homebrew
/usr/bin/ruby -e "$(curl -fsSL https://raw.githubusercontent.com/Homebrew/install/master/install)"
02 - Run a few cleanup and diagnostics
03 - Now you can install ImageMagick
brew install imagemagick
How to use ImageMagick?
01 - Open your terminal app.
02 - Inside the terminal go to the folder where your images are
Note: after typing
cd (change directory) you can drag'n drop your folder from your Finder window to help you type the full path.
03 - Merge vertically your different screenshots
convert -append pomodrone*.png pomodrone.png
-append is the function to merge vertically your images. You can use
+append to merge them horizontally. Of course there's tons of more options http://www.imagemagick.org/script/command-line-options.php#append
Et voilà, the result
pomodrone.png is in the same folder.
All I wanted to demonstrate here is that lots of apps are just simple GUI applying commands for you. By learning them you'll save lots of money and use the app directly like you want at a blazing speed.
Anyway, I won't disturb you on that subject anymore and sorry if it was a bit hard.