I understand my title makes no sense, so I’ll explain.
I have seen countless games where they use parts of their game (such as maps, lobbies, or any other 3d objects) in their thumbnails and game icons.
I am interested in doing the same thing for my games thumbnails and icon, but I do not know how to approach this.
I guess my questions is:
How do a get the photo of the map/part of the game I want for the image?
Sorry if any of this is vague. 
Thank you for any help that is given! 
Shift + P in-game will allow you to enable a freecam. It gets rid of all the UI and lets you take great pictures of your game, and even comes with the ability to change the camera FOV.
There are various ways this can be approached. Most people use screenshots of the game and add some flair in photo editing software. Other people commission artists to draw or render thumbnails for them. Screenshots can be saved with Windows’ snipping tool or the PrintScreen key.
Renders can be especially useful if you want to highlight certain objects (in thumbnails for example). Designers often render cars, pets, new items, important characters etc. to get a high resolution image that can later be used in compositions.
People sometimes use renders for complete in-game scenes (i.e. lobbies as you mentioned), but personally I prefer @LocksOfDispair’s method since it gives your users a more honest view of your game and is generally easier and takes less time.
I think they changed it to Shift+Ctrl+P:
It works both ways for me, but I am on a mac, so.