Strictly speaking, the Bitmap shader is not a shader because it actually loads an image. For reasons of consistency, it is nevertheless classified as a shader. Contrary to real shaders, it does not calculate a texture procedurally but makes a bitmap available to the material. Take a look at the Textures section, which offers a great deal of useful information.

At the top of the dialog window you will find three buttons with the following commands:

If you edit an image using an external application, it can be updated in Cinema 4D by clicking on the Reload Image button. Clicking on Edit Image will open the image in an external image editing application, as defined by your operating system. Locate Image … opens the image in your operating system’s Explorer/Finder