WebGL - Upload photo to game from PC

You would need to handle it the same way you handle all user uploads to your server. You need an HTML Form for submitting the images, and a server that can handle uploads. Optionally, you could use something like SFTP if your just collecting textures from your team.

When you store the files, make sure they are in a readable directory. Then you can access them with:

 public string url = "http://yourserver.com/uploads/webcamTexture.jpg";
     IEnumerator Start() {
         WWW www = new WWW(url);
         yield return www;
         Renderer renderer = GetComponent<Renderer>();
         renderer.material.mainTexture = www.texture;
     }

Docs for the WWW class.