Game art outsourcing is a common practice of transferring design and art work on a project to another company, or, more correctly, a game art studio, which specializes in this area and takes responsibility for completing the agreed work under the contract.
You can transfer the project to a contractor team in full so that the selected studio completes the project on a turnkey basis from start to release and transfers the final source files for further distribution of the product, or partially, allocating a limited amount of work, which, for example, has certain specifics or requires a special list of skills.
A common practice is when the customer company specializes only in art and design, so it outsources programming and integration to another team that specializes in development. Or, conversely, the company only has an engineering department and programming expertise, so game art is outsourced to a team that specializes in graphics development.
Companies such as Fgfactory are fully specialized in outsourcing services and perform a full range of tasks for the development of mobile, browser and desktop games including 2D, 3D graphics, animation, interface, programming of game engines, server development, testing and support.