diff --git a/aider/coders/base_coder.py b/aider/coders/base_coder.py
index 8c3484c69..55ca97f38 100755
--- a/aider/coders/base_coder.py
+++ b/aider/coders/base_coder.py
@@ -1,5 +1,6 @@
 #!/usr/bin/env python
 
+import base64
 import hashlib
 import json
 import locale
@@ -652,9 +653,11 @@ class Coder:
         image_messages = []
         for fname, content in self.get_abs_fnames_content():
             if is_image_file(fname):
+                with open(fname, "rb") as image_file:
+                    encoded_string = base64.b64encode(image_file.read()).decode('utf-8')
                 mime_type, _ = mimetypes.guess_type(fname)
                 if mime_type and mime_type.startswith("image/"):
-                    image_url = f"data:{mime_type};base64,{content}"
+                    image_url = f"data:{mime_type};base64,{encoded_string}"
                     rel_fname = self.get_rel_fname(fname)
                     image_messages += [
                         {"type": "text", "text": f"Image file: {rel_fname}"},