C++/OpenCV - How to index pictures for image retrieval? (and find the best image)

Question

I would like to use BoW with FLANN in order to index all my pictures inside my database.(For a CBIR project)

What i did :

1. Loop over all ma database in order to compute the descriptors of each pictures.
2. Clustering descriptors using K-MEAN
3. Extraction of BoWDescriptors (Visual words) in order to have a big histogram with all my features
4. Use FLANN Index to compute an index in each pictures

Code:

// Create Flann LSH index
cv::flann::Index flannIndex(this->descDescriptorbow, cv::flann::LshIndexParams(12, 20, 2), cvflann::FLANN_DIST_HAMMING);

cv::Mat results, dists;

int k=2; // find the 2 nearest neighbors

// search (nearest neighbor)
flannIndex.knnSearch(responseHist, results, dists, k, cv::flann::SearchParams() );

DescriptorBow contains all my BoWfeatures of my database. ResponseHist constains the bowFeature of my picture i want to eval.

Now, what i have to do in order to see what is the best matching picture ?

Can someone help me ?