diff --git a/fotogrid.py b/fotogrid.py
index 321b4df..9282856 100644
--- a/fotogrid.py
+++ b/fotogrid.py
@@ -1,22 +1,32 @@
+# SPDX-License-Identifier: GPL-3.0
+
+
 def fotogrid(image_urls):
     """
     Provide a simple responsive fotogrid using flex layout.
     Depends on the css/fotogrid.css defined styles.
+    Use jinja {{ fotogrid([....]) }} from templates (registered in app.py)
     """
-    result = '<div class="fotorow">'
-    cols = 4
-    images = divmod(len(image_urls), cols)
-    images_per_col = images[0]
-    if images[1] > 0:
-        images_per_col += 1
-    nbr = 0
-    for img in image_urls:
-        if (nbr % images_per_col) == 0:
-            if nbr > 0:
-                result += '</div>'  # closing fotocolumn
-            result += '<div class="fotocolumn">'
-        result += '<img src="' + img + '" style="width:100%">'
-        nbr += 1
+    if len(image_urls) == 0:
+        return ""
+
+    # prepare our 'bins', the columsn of the fotogrid
+    nbr_of_columns = 4
+    prepared_column = []
+    for i in range(nbr_of_columns):
+        prepared_column.append([])
+    # sort given image URL into bins
+    for i in range(len(image_urls)):
+        prepared_column[i % nbr_of_columns].append(image_urls[i])
+
+    # form HTML div structure from prepared bins
+    html = '<div class="fotorow">'
+    for column in prepared_column:
+        if len(column) > 0:
+            html += '<div class="fotocolumn">'
+            for url in column:
+                html += '<img src="' + url + '" style="width:100%">'
+            html += "</div>"  # closing fotocolumn
+    html += "</div>"  # closing fotorow
 
-    result += "</div></div>"  # closing fotocolum, fotorow
-    return result
+    return html
diff --git a/tests/test_fotogrid.py b/tests/test_fotogrid.py
new file mode 100644
index 0000000..5a34103
--- /dev/null
+++ b/tests/test_fotogrid.py
@@ -0,0 +1,38 @@
+# SPDX-License-Identifier: GPL-3.0
+import os
+
+from fotogrid import  fotogrid
+from lxml import etree
+from io import StringIO
+
+def check_if_valid_html(html):
+    parser = etree.HTMLParser(recover=False)
+    etree.parse(StringIO(html), parser)
+    assert len(parser.error_log) == 0
+
+
+def test_fotogrid_empty_list():
+    urls = []
+    html = fotogrid(urls)
+    assert len(html) == 0
+
+
+def check_if_images_result_in(nbr_of_images, rows_expected, cols_expected):
+    urls = ["dummy.jpg"] * nbr_of_images
+    html = fotogrid(urls)
+    check_if_valid_html(html)
+    assert html.count("fotorow") == rows_expected
+    assert html.count("fotocol") == cols_expected
+    assert html.count("<img ") == nbr_of_images
+
+
+def test_fotogrid(app, client):
+    check_if_images_result_in(1, 1, 1)
+    check_if_images_result_in(2, 1, 2)
+    check_if_images_result_in(3, 1, 3)
+    check_if_images_result_in(4, 1, 4)
+    check_if_images_result_in(5, 1, 4)
+    check_if_images_result_in(6, 1, 4)
+    check_if_images_result_in(7, 1, 4)
+    check_if_images_result_in(8, 1, 4)
+    check_if_images_result_in(9, 1, 4)
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