Fix image loading.

test/requirements.txt

@@ -0,0 +1 @@
+pytest

test/test_img_replace.py

@@ -0,0 +1,14 @@
+import markdown 
+from bs4 import BeautifulSoup
+
+md_input = """This is an image of a tree: ![tree image](tree_img)
+"""
+
+def test_image_replace():
+
+    soup = BeautifulSoup(markdown.markdown(md_input), features="html.parser")
+
+    for img in soup.find_all('img'):
+        img["src"] = "blah"
+
+    print(soup.prettify())