You have to have tesseract installed and accesible in your path.

According to source, pytesseract is merely a wrapper for subprocess.Popen with tesseract binary as a binary to run. It does not perform any kind of OCR itself.

Relevant part of sources:

def run_tesseract(input_filename, output_filename_base, lang=None, boxes=False, config=None):
    '''
    runs the command:
        `tesseract_cmd` `input_filename` `output_filename_base`

    returns the exit status of tesseract, as well as tesseract's stderr output
    '''
    command = [tesseract_cmd, input_filename, output_filename_base]

    if lang is not None:
        command += ['-l', lang]

    if boxes:
        command += ['batch.nochop', 'makebox']

    if config:
        command += shlex.split(config)

    proc = subprocess.Popen(command,
            stderr=subprocess.PIPE)
    return (proc.wait(), proc.stderr.read())

Quoting another part of source:

# CHANGE THIS IF TESSERACT IS NOT IN YOUR PATH, OR IS NAMED DIFFERENTLY
tesseract_cmd = 'tesseract'

So quick way of changing tesseract path would be:

import pytesseract
pytesseract.tesseract_cmd = "/absolute/path/to/tesseract"  # this should be done only once 
pytesseract.image_to_string(img)