Snapmatic is basically a camera app that lets you take pictures in game via your phone (so a normal camera phone :P) which was introduced with the old-gen (Xbox 360/Playstation 3) version back in 2013, back then the photos were in 640x360 resolution. With the current-gen (Xbox One/Playstation 4) release the pictures became high definition, and they reached a resolution of 960x536. The current PC version also features the same 960x536 resolution for Snapmatic pictures, which is the reason why I would recommend recording clips from the game and editing the camera angles, effects, DoF and so on in the Rockstar Editor and then taking a picture with the Screenshot button on Steam, but of course not everybody wants to spend that much time on just a couple of screenshots!

You are also able to add effects, borders, zoom in and out and of course move the camera around.
Other features such as taking a selfie and locking the focus on certain target can also be used.

The app is available in GTA Online (multiplayer) as well as in GTA V (singleplayer) for all of the characters.

Okay, so let's get started then:
The first thing that you want to do is hit the Up Arrow key on your keyboard, from there move to the left bottom corner where the Snapmatic app is located in and hit Enter



Now, the app should've opened.
From here you can change the Borders, Filters, Zoom in and out, move the camera around the picture and so on



After you're happy with the camera angles, and so on, hit Enter and it will take the picture



Hit Enter again to save the picture to your Gallery



Now, hit Esc on your keyboard to open the Pause Menu, keep moving to the right to find 'Gallery' and hit Enter.
In there, you can see all of the pictures that you've taken with Snapmatic



By pressing Enter on the picture you just took, you can change the title and also add a meme text



By pressing R Ctrl (Right Ctrl) on your keyboard the game will ask you if you are sure that you want to upload the picture to Social Club. Press Enter to upload the picture to Social Club

After you have uploaded your picture from the ingame Gallery to Social Club, go to http://socialclub.rockstargames.com/games/gtav/pc/snapmatic and log in.
You can also find the Snapmatic button on your own profile on Social Club, if you want to enter it that way

Once you have logged in, you will be loaded in to the Snapmatic gallery page, you can switch the platform by clicking the [PC] Social Club button on the top right, but I'm guessing you've taken the picture on PC so I already changed the platform to PC on the link.
From here, click on the 'Most Recent' tab next to the search box, from there choose 'My Photos'



Now you are browsing your own Social Club pictures



By clicking the picture with your mouse, you can open it up and from there you can download it on to your PC for you to upload it to Facebook, Steam, Twitter, etc

Well that was the guide on how to take Snapmatic pictures ingame with the app!

In my opinion, the part where you have to go to Social Club and download the pictures from there to your PC, should be replaced by just letting the players choose between uploading them to Social Club, saving them on their HDD's or just allowing the players to do both at the same time, but oh well, the same method has been in the game since it was released, so I doubt they will change it now!

Anyway, I hope you found the guide helpful!

Also, here's the full picture that I took, in case somebody is interested :)