There are many singular English nouns that end with an “s”, such as mathematics, physics, or politics. Those are typically names of certain fields of science or human activity.
The word “graphics” fits the above pattern. It can be used in the singular to refer to the field of graphic art or the process of making graphics:
There is, however, one important difference. While there is no “one mathematic” and “one physic”, there in fact is “one graphic”. A graphic, apart from its traditional meaning of a drawing or a picture used as an illustration in a book or a newspaper, can also refer to a computer-generated image viewed on a screen. And here’s the catch.
When “graphic” refers to a computer-generated image, it is usually used in the plural (since what you see on your computer screen is a sequence of images, not just a static image). When people say that a computer game has good graphics, the intended meaning is usually the plural of “graphic”. In other words, most people would say:
On the other hand, some people refer to the process via which the graphics were generated, which would be the singular graphics:
Such a sentence should be understood as “this game’s process of generating what we see is really good”, which also makes perfect sense. Be that as it may, it is much less common than “graphics are”, to such a degree that many native speakers would consider it to be an error. Hence, it is advisable to use the plural when speaking about a game’s graphics.