生成した画像を表示できない

pythonとflaskで作ったモザイクアートを生成するプログラムを作りました。
生成した画像をhtmlで表示させたいのですが、
FileNotFoundError: [Errno 2] No such file or directory: './tmp/output.jpg'
と出てしまいます。
なぜなのでしょうか。
url:https://gentle-savannah-92123.herokuapp.com/

Python
1from flask import Flask, render_template, request
2from PIL import Image,ImageFilter
3import subprocess
4import os
5import random
6import math
7import time
8import numpy as np
9import glob
10import cv2
11
12app = Flask(__name__)
13
14# getのときの処理
15@app.route('/', methods=['GET'])
16def get():
17    return render_template('index.html', \
18        title = 'モザイクアートを作ろう', \
19        message = '設定してください')
20
21# postのときの処理
22@app.route('/', methods=['POST'])
23def post():
24    SourceDir = request.form['source']
25    GoalImage = request.form["goal"]
26    OutputImage="output.jpg"
27    SourceImageSize=(60,40)
28
29    TargetZoom=11
30
31    used_count = 3
32
33
34    def __create_tile(org_image,height,width):
35
36        w, h = org_image.size
37        RectList=[]
38        for y in range(0, math.floor(h / height) + 1):
39            for x in range(0, math.floor(w / width) + 1):
40                height2 = y * height
41                width2 = x * width
42                crap_img = org_image.crop((width2, height2, width2 + width, height2 + height))
43                RectList.append((crap_img,width2,height2))
44        return RectList
45
46    def __clac_hist(img):
47
48        hist_list = []
49        color = ['r','g','b']
50        images = np.asarray(img)
51
52        for i in enumerate(color):
53            hist_list.append(cv2.calcHist([images],[i[0]],None,[256],[0,256]))
54        return hist_list
55
56    if __name__=="__main__":
57
58        startTime=time.time()
59
60
61        target=Image.open(GoalImage)
62        target=target.resize(tuple(math.floor(i*TargetZoom) for i in target.size))
63
64
65        tiles = __create_tile(target,SourceImageSize[1],SourceImageSize[0])
66
67        file_paths = glob.glob(SourceDir+"\*")
68        src_hist_dict = {}
69        for file_path in file_paths:
70            file_name = file_path.split("\")[-1]
71
72            try:
73                src_image =  Image.open(file_path).resize(SourceImageSize)
74            except Exception:
75                continue
76            src_hist_dict[file_name] = [src_image,__clac_hist(src_image),0]
77
78
79        Target=Image.new("RGB",target.size,255)
80        print("モザイクアート生成開始")
81        while(len(tiles) > 0):
82            result = []
83
84            r=random.randrange(len(tiles))
85            tileRect=tiles[r]
86            del tiles[r]
87
88            tile_hist = __clac_hist(tileRect[0])
89
90            tile_hist = np.array(tile_hist)
91            tile_hist = tile_hist.reshape(tile_hist.shape[0] * tile_hist.shape[1], 1)
92
93
94            for file_name,src_hist in src_hist_dict.items():
95                src_hist = np.array(src_hist[1])
96                src_hist = src_hist.reshape(src_hist.shape[0] * src_hist.shape[1], 1)
97
98                d = cv2.compareHist(tile_hist, src_hist, cv2.HISTCMP_INTERSECT)
99                result.append([d,file_name])
100            result.sort(reverse=True)
101
102            Target.paste(src_hist_dict[result[0][1]][0],(tileRect[1],tileRect[2]))
103
104            src_hist_dict[result[0][1]][2] = src_hist_dict[result[0][1]][2] + 1
105
106        OutputImage="./tmp/output.jpg"
107        Target.save(OutputImage)
108        Target.show()
109    return render_template("index.html",message="モザイクアート生成完了", image_url=OutputImage)
110
111
112if __name__=="__main__":
113    app.run(debug=True)
114

index.html
1<!DOCTYPE html>
2<html lang="jn" dir="ltr">
3    <head>
4        <meta charset="utf-8">
5        <title>モザイクアート生成</title>
6    </head>
7    <body>
8        <h1>{{ title }}</h1>
9<p>{{ message }}</p>
10<form action="/" method="POST" enctype="multipart/form-data">
11    <div>
12        <label for="name">素材の入ったフォルダ：</label>
13        <input type="text" id="source" name="source" placeholder="C:xxxx">
14    </div>
15    <div>
16        <label for="name">目標画像：</label>
17        <input type="text" id="goal" name="goal" placeholder="C:xxxx">
18    </div>
19    <div>
20        <input type="submit" value="モザイクアート生成">
21    </div>
22</form>
23        <img src="{{ image_url }}" />
24    </body>
25</html>
26