As you're working on your thumbnail layouts you need to consider the shot-- like a film-maker/cinematographer-- looking for the best shot takes a but of imagination and understanding. Studying film is one of the best ways to improve your skills as a graphic novelist or comic book artist.
Let's take a look at these examples; Say you want to show a city.
Your first thought should be-- what is a city? Is it a modern city-- is it an old gothic style city? Seaside, inland, all that jazz. Once you've determined the type of city you have to consider the shot-- what angle do you want to use? In this first example we clearly see that it's a fairly large city. The big buildings let us know we're not in Smallville here.
But if you set your shot at what's called a "worm's eye view" you will give your reader the impression of the height and density of this city. Going beyond the first shot that comes to your mind-- in this case a straight on shot of the buildings and considering a worm's eye view will improve your storytelling by helping your reader "feel" what's happening in your story.
Closeups give us a sense of what the character is feeling. In this first example we have a Medium Shot-- usually encompassing the character and enough of it's element that we have both a setting and an environment-- in this case a restaurant.
A Closeup brings us in tighter to the subject-- giving the reader a feeling of intimacy towards the subject matter. Closeups should be used only after a medium shot has established a characters placement in a given situation, otherwise it will confuse your reader. It can be placed first and then a later panel reveals the medium shot if you want to get fancy.
Extreme Closeups are used when we want to get inside a character's thoughts or emotions. Again, best used after we've already established location and situation.