Do you mean like a thumbnail image?
For that, I’d recommend using any drawing program that has separate layers, like GIMP or Krita. I’d avoid Microsoft Paint since everything is on one layer. The reason you want layers is so that you can separate different elements of your image, and more easily swap out individual elements to see how they look.
I’m not an expert, but if you’re aiming for a good-looking thumbnail, I think there are two key elements that are vital to a good thumbnail: your title, and the background art. First up with the title, you want it to be fairly large and easy to see at a glance. If you have the time, make it something unique and stylized. No matter what though, you should make sure it’s easily readable at a glance. Next with the background art, you can either just take a screenshot of your game and crop it, or you can make a special illustration specifically for the thumbnail. Either way, it should represent the game. If the main character of your game is a cute little kitty, for example, then you better show that cat on the cover. Better yet, if your game is also a platformer, you can show that by making it so the cat is jumping from one platform to another. This communicates to the viewer the theme and genre of your game.
These tips aren’t absolutely necessary for your first game, but if you’re really wanting people to click on it and check it out, then it’s a good idea to have a thumbnail that shows off your game in some capacity.