How to change textures after game is built?

I would like to build a game which is able to change it textures after it is built and run (for windows platform would be enough). My point is to avoid handing over the unity project containing the source code to my designer. I want to hand over a build version of the game to the designer, but at the same time, I want him to be able to change in game textures at any time.

Is this possible? Can someone lead me to some information?

Loading textures from file will help you solve this issue, look at this post :

Post

which will guide you on how to load textures from file, however I suggest that you add a system that checks IF the file exists first because then you can have a fail-safe for when the texture file is deleted or missing that will either create a new file or maybe read from textures built with the game.

You could hand over a project with compiled DLLs instead of source code. Your designer will probably need to do more than add textures, such as creating materials, setting the resolution of the textures, etc. To create DLLs: Unity - Manual: Managed plug-ins

As an added bonus, projects will build faster because they don’t have to recompile source code. So your designer will be able to iterate faster.