In our project we use wide strings (std::wstring + wchar_t* ) by default - so it's more compatible to file names with locale symbols (cyr) and it's easy to integrate with .NET marshalling.

In our OpenCV code we use something like this:

    void load(wstring filename){
        Mat img = imread(string(filename.begin(), filename.end()), CV_LOAD_UNCHANGED);
    } 
   void load(wchar_t* filename){
    wstring wfname(filename);
        Mat img = imread(string(wfname.begin(), wfname.end()), CV_LOAD_UNCHANGED);
    } 

Our suggestion - supply overloads (or template) for imread/imwrite to use std::wstring, wchar_t*

Better if in nuts it will be wide char based, and std::string version - as overload.