diff --git a/avatar/conf.py b/avatar/conf.py
index bb25c4f..1d94d6e 100644
--- a/avatar/conf.py
+++ b/avatar/conf.py
@@ -17,6 +17,7 @@ class AvatarConf(AppConf):
     MAX_SIZE = 1024 * 1024
     THUMB_FORMAT = "PNG"
     THUMB_QUALITY = 85
+    THUMB_MODES = ["RGB", "RGBA"]
     HASH_FILENAMES = False
     HASH_USERDIRNAMES = False
     EXPOSE_USERNAMES = False
diff --git a/avatar/models.py b/avatar/models.py
index b915a75..6aa20d6 100644
--- a/avatar/models.py
+++ b/avatar/models.py
@@ -163,8 +163,8 @@ class Avatar(models.Model):
                 else:
                     diff = int((h - w) / 2)
                     image = image.crop((0, diff, w, h - diff))
-                if image.mode not in ("RGB", "RGBA"):
-                    image = image.convert("RGB")
+                if image.mode not in (settings.AVATAR_THUMB_MODES):
+                    image = image.convert(settings.AVATAR_THUMB_MODES[0])
                 image = image.resize((size, size), settings.AVATAR_RESIZE_METHOD)
                 thumb = BytesIO()
                 image.save(thumb, settings.AVATAR_THUMB_FORMAT, quality=quality)
diff --git a/docs/index.txt b/docs/index.txt
index e36b250..cd652b9 100644
--- a/docs/index.txt
+++ b/docs/index.txt
@@ -222,6 +222,13 @@ appear on the site. Listed below are those settings:
     The quality of thumbnails, between 0 (worst) to 95 (best) or the string
     "keep" (only JPEG) as provided by Pillow. Defaults to `85`.
 
+.. py:data:: AVATAR_THUMB_MODES
+    A list of acceptable modes for thumbnails as provided by Pillow. If the mode
+    of the image is not in the list, the thumbnail will be converted to the
+    first mode in the list. Note that you need to set this to modes available
+    for AVATAR_THUMB_FORMAT and JPEG does not support RGBA. Defaults to
+    `['RGB', 'RGBA']`.
+
 .. py:data:: AVATAR_CLEANUP_DELETED
 
     ``True`` if the avatar image files should be deleted when an avatar is