diff --git a/lib/src/client.dart b/lib/src/client.dart
index b667ef3..e4ecdcd 100644
--- a/lib/src/client.dart
+++ b/lib/src/client.dart
@@ -488,7 +488,7 @@ class Client {
 
   /// Uploads a new user avatar for this user.
   Future<void> setAvatar(MatrixFile file) async {
-    await file.resize(width: 128);
+    await file.resize(width: Client.defaultThumbnailSize);
     final uploadResp = await upload(file);
     await jsonRequest(
         type: HTTPType.PUT,
@@ -557,6 +557,7 @@ class Client {
   static const List<String> supportedGroupEncryptionAlgorithms = [
     'm.megolm.v1.aes-sha2'
   ];
+  static const int defaultThumbnailSize = 256;
 
   http.Client httpClient = http.Client();
 
diff --git a/lib/src/room.dart b/lib/src/room.dart
index 856dc64..965ffe3 100644
--- a/lib/src/room.dart
+++ b/lib/src/room.dart
@@ -503,10 +503,10 @@ class Room {
 
     if (msgType == 'm.image') {
       var thumbnailPathParts = file.path.split('/');
-      thumbnailPathParts.last = 'thumbnail_' + thumbnailPathParts.last;
+      thumbnailPathParts.last = 'thumbnail_' + thumbnailPathParts.last + '.jpg';
       final thumbnailPath = thumbnailPathParts.join('/');
       thumbnail = MatrixFile(bytes: file.bytes, path: thumbnailPath);
-      await thumbnail.resize(width: 512);
+      await thumbnail.resize(width: Client.defaultThumbnailSize);
       fileImage = decodeImage(file.bytes.toList());
       thumbnailImage = decodeImage(thumbnail.bytes.toList());
     }
@@ -1250,7 +1250,7 @@ class Room {
   /// Uploads a new user avatar for this room. Returns the event ID of the new
   /// m.room.avatar event.
   Future<String> setAvatar(MatrixFile file) async {
-    await file.resize(width: 256);
+    await file.resize(width: Client.defaultThumbnailSize);
     final uploadResp = await client.upload(file);
     final setAvatarResp = await client.jsonRequest(
         type: HTTPType.PUT,
diff --git a/lib/src/utils/matrix_file.dart b/lib/src/utils/matrix_file.dart
index 15c5347..fd4a57a 100644
--- a/lib/src/utils/matrix_file.dart
+++ b/lib/src/utils/matrix_file.dart
@@ -12,6 +12,7 @@ class MatrixFile {
   /// If this file is an Image, this will resize it to the
   /// given width and height. Otherwise returns false.
   /// At least width or height must be set!
+  /// The bytes will be encoded as jpg.
   Future<bool> resize({int width, int height}) async {
     if (width == null && height == null) {
       throw ('At least width or height must be set!');
@@ -23,7 +24,7 @@ class MatrixFile {
       return false;
     }
     final resizedImage = copyResize(image, width: width, height: height);
-    bytes = encodePng(resizedImage);
+    bytes = encodeJpg(resizedImage, quality: 10);
     return true;
   }