There are lots of free and paid software around that we can use to create game graphic, I particular like these three softwares, Blender, Inkscape and GIMP. I use Blender to create 3D game graphic, use Inkscape to create 2D game graphic and use GIMP to draw simple graphic which I can then edit on Inkscape and Blender.
GIMP is pretty easy to master and you can start using it after a week of training, gimp is use more on photo editing job but creating simple graphic is also not bad at all.
Inkscape is use mainly in creating 2d game graphic, it has lots of features so it will take a while for a beginner to master it.
Blender is a powerful graphic creation software which we can use to create 2d, 3d as well as gaming landscape for our computer game. Blender creates graphic which is close to reality and is a must for game developer to get this tool and starts to create great product.
Game creation takes time and lots of brain energy, if you are a game developer then you must good at both coding and graphic creation. Those three softwares above are just the beginning to your game development journey because as time goes by you might even need to learn autocad or other sophisticated computer graphic generation software which will cost money, therefore game developing business is not an easy business after all but it is fun when you see you game being plays by many users online! That just make you feel happy about it!