The Xbox cannot handle forward slashes in paths. You must provide a path with a backslash separator. In C/C++ a backslash character must be escaped and the escape character is a backslash so the result is two per: "\\". It is also good to use the drive letter which is D: in the case of your applications working directory. Using D: ensures that your application can be executed from any installation location. So try the following snippet:
CODE
//Loading Bitmap
#ifdef _XBOX
Padel = load_image("D:\\sprite_one.bmp");
#else
Padel = load_image("image/sprite_one.bmp");
#endif
In a large cross-platform application it is good to use preprocessor directives to define a global const that can be used where needed so that you don't need preprocessor directives all over the code.
CODE
#if (defined _WIN32 || defined _XBOX)
#define PATHSEP "\\"
#define PATHSEPCHAR '\\'
#else
#define PATHSEP "/"
#define PATHSEPCHAR '/'
#endif