GIMP is a free cross-platform image editor available for GNU/Linux, OS X, Windows and more operating systems. 

This software is a free alternative to photoshop. I mostly use it to crop and resize images.

To my surprise Adobe XD is completely free to use. Adobe XD is a powerful, collaborative, easy-to-use platform that helps you and your team create designs for websites, mobile apps, voice interfaces, games, and more.

 

Inkscape is free professional quality vector graphics software which runs on Linux, Mac OS X and Windows desktop computers. This software can be used to create or edit vector graphics such as illustrations, diagrams, line arts, charts, logos and complex paintings.

Krita is a professional FREE and open source painting program. It is made by artists that want to see affordable art tools for everyone.

  • Concept art
  • Texture and matte painters
  • Illustrations and comics

I recommend using drawing tablets (wacom products, surface book, tablet laptops, etc.)

Draw lets you produce anything from a quick sketch to a complex plan, and gives you the means to communicate with graphics and diagrams. With a maximum page size of 300cm by 300cm, Draw is a an excellent package for producing technical drawings, brochures, posters and more. Draw lets you manipulate graphical objects, group them, crop them, use objects in 3D and much more.