Python pool.map for list of tuples

I've got a following issue. I'm trying to refactor my code in order to process API calls using multithreading. My core data is simple list of tuples in following format:

lst = [('/Users/sth/photo1.jpg',
      '/Users/sth/photo2'),
      ('/Users/sth/photo1.jpg',
      '/Users/sth/photo3'), (...)]

Function that I use takes lst list and process it through an API which requires a pair of photos. After all a single number is returned for each pair. So far, I'm using a loop to put a tuple into my function and produce mentioned number. I would like to paralellize the whole computation in a way that one process takes a part of my list and calls the function for the tuples inside a batch. To do that I was trying to use pool function for multiprocessing module:

from multiprocessing.dummy import Pool as ThreadPool 
pool = ThreadPool(2)
results = pool.map(score_function, lst)

However, following error occurs:

IOError: [Errno 2] No such file or directory: 'U'

Something strange is happening here. It tries to treat a single character from my tuple as an argument. Any ideas how to do it properly?

Thank You

@edit

The lack of score_function definition is my bad. Let me update the question:

def score_function(pairs):
    score_list = list()

    for pair in pairs:
       score = findElement(target = pair[0], source = pair[1])
       score_list.append([pair[0], pair[1], score])

    return score_list

Where findElement is defined as:

def findElement(target, source):

    with open(source, 'rb') as source_:
        source_bytes = source_.read()

    with open(target, 'rb') as target_:
        target_bytes = target_.read()

    score = API_request(target_bytes = target_bytes,
                        source_bytes = source_bytes)
    return score