All I wanted to do was check if my application was running in debug-mode or in release-mode. Searching MSDN library and Google (groups) both did not deliver the solution to me. I was about to let go when, out of the blue, a webpage showed up in my browser telling me the following:

Visual Studio automatically generates a pre-processor directive called DEBUG which is linked to the mode you run your application in. Beware: these directives are case sensitive…

This meant an easy solution for my ‘problem’!

Pre-processor directives can be defined using #define (making the directive true) and #undef (making it false) in C#. When you check one of these directives using an #if … #else … #endif statement Visual Studio even greys out the code that will not be compiled, because of the directives. This gives you a pretty good idea which code will and which code will not be compiled into the final assembly.
Because of the pre-processor directive behaviour, you can easily create cross-platform applications by simply changing one directive. Changing that one directive may change complete pieces of code in your application.

The answer to my question was simple:

#if (DEBUG)
    imgPath = GetDirectoryName(Application.ExecutablePath);
#else
    imgPath = GetDirectoryName(Application.ExecutablePath) + "\images\";
#endif