It is easy enough to create a separate class for the Py2 version.

src/img2pdf.py

@@ -29,6 +29,8 @@ from enum import Enum
 from io import BytesIO
 import logging
 
+PY3 = sys.version_info[0] >= 3
+
 __version__ = "0.2.3"
 default_dpi = 96.0
 papersizes = {
@@ -269,6 +271,7 @@ class MyPdfWriter():
         self.addobj(page)
 
 
+if PY3:
     class MyPdfString():
         @classmethod
         def encode(cls, string):
@@ -280,6 +283,15 @@ class MyPdfString():
             string = string.replace(b'(', b'\\(')
             string = string.replace(b')', b'\\)')
             return b'(' + string + b')'
+else:
+    class MyPdfString(object):
+        @classmethod
+        def encode(cls, string):
+            # This mimics exactely to what pdfrw does.
+            string = string.replace(b'\\', b'\\\\')
+            string = string.replace(b'(', b'\\(')
+            string = string.replace(b')', b'\\)')
+            return b'(' + string + b')'
 
 
 class pdfdoc(object):