rgb = imread(filename) ;
mask = rgb(:, 1) < 32 & rgb(:, 2) > 224 & rgb(:, 3) > 224;
mask = bwareafilt(mask, [50 inf]);
props = regionprops(mask, 'centroid') ;
locations = vertcat(props.Centroid);
The constants in the above are not tested, only "by eye", and probably need to be adjusted.
The resulting variable will be a something by 2 array in which the first column is x and the second is y. Note that x corresponds to columns, second array index, and y corresponds to row, first index.