diff --git a/internal/storage/oss.go b/internal/storage/oss.go
index 1517dfa..4839877 100644
--- a/internal/storage/oss.go
+++ b/internal/storage/oss.go
@@ -8,10 +8,8 @@ import (
 	"fmt"
 	"io"
 	"mime/multipart"
-	"time"
 
 	"github.com/pkg/errors"
-	"github.com/thanhpk/randstr"
 	"github.com/wuhan005/gadget"
 
 	"github.com/tamakyi/TamaBox/internal/conf"
@@ -19,7 +17,7 @@ import (
 
 // UploadPictureToOSS upload user's avatar or background image to OSS.
 // It returns the uploaded asset URL.
-func UploadPictureToOSS(file multipart.File, _ *multipart.FileHeader) (string, error) {
+func UploadPictureToOSS(file multipart.File, fileHeader *multipart.FileHeader) (string, error) {
 	// 读取文件头，校验是否为图片
 	header := make([]byte, 512)
 	n, err := file.Read(header)
@@ -43,12 +41,12 @@ func UploadPictureToOSS(file multipart.File, _ *multipart.FileHeader) (string, e
 		return "", errors.Wrap(err, "get OSS bucket")
 	}
 
-	now := time.Now()
-	year := now.Year()
-	month := int(now.Month())
-	day := now.Day()
-
-	key := fmt.Sprintf("%s%d/%02d/%02d/%s", PictureKeyPrefix, year, month, day, randstr.Hex(15))
+	// OSS 通道原样上传原图（不转 WebP），扩展名沿用原始文件名。
+	ext := ""
+	if fileHeader != nil {
+		ext = originalExt(fileHeader.Filename)
+	}
+	key := uploadKey(ext)
 
 	if err := gadget.Retry(5, func() error {
 		if err := bucket.PutObject(key, file); err != nil {
diff --git a/internal/storage/picture.go b/internal/storage/picture.go
index 02c81b6..8f4d8b5 100644
--- a/internal/storage/picture.go
+++ b/internal/storage/picture.go
@@ -4,7 +4,46 @@
 
 package storage
 
-const PictureKeyPrefix = "picture/"
+import (
+	"fmt"
+	"path/filepath"
+	"strings"
+	"time"
+)
+
+// PictureKeyPrefix 是图片对象 key 的前缀。生产环境图片以无前缀方式存于
+// bucket 根下（R2 PathStyle 下 bucket 名自动作为 URL 首段），故这里置空。
+const PictureKeyPrefix = ""
+
+// uploadKey 按生产口径生成对象 key：`YYYY/MM/<UnixNano><ext>`。
+// 无前缀、无 day 层，文件名用纳秒时间戳保证唯一，扩展名与最终落盘内容类型一致。
+func uploadKey(ext string) string {
+	now := time.Now()
+	return fmt.Sprintf("%s%d/%02d/%d%s", PictureKeyPrefix, now.Year(), int(now.Month()), now.UnixNano(), ext)
+}
+
+// extFromContentType 由 MIME 类型推导扩展名，用于保证对象 key 的扩展名
+// 与 ContentType（以及实际落盘字节）严格一致，避免出现「.jpg 实为 webp」的错配。
+func extFromContentType(contentType string) string {
+	switch contentType {
+	case "image/jpeg":
+		return ".jpg"
+	case "image/png":
+		return ".png"
+	case "image/gif":
+		return ".gif"
+	case "image/webp":
+		return ".webp"
+	default:
+		return ""
+	}
+}
+
+// originalExt 取原始文件名的扩展名，统一转小写。用于转换失败回退原图时
+// 保留原始扩展名（与生产口径一致）。
+func originalExt(filename string) string {
+	return strings.ToLower(filepath.Ext(filename))
+}
 
 // MaxAvatarSize is the max avatar size which is 2MB.
 const MaxAvatarSize = 2 * 1024 * 1024
diff --git a/internal/storage/picture_test.go b/internal/storage/picture_test.go
new file mode 100644
index 0000000..56343f7
--- /dev/null
+++ b/internal/storage/picture_test.go
@@ -0,0 +1,46 @@
+package storage
+
+import (
+	"regexp"
+	"testing"
+)
+
+// 生产口径 key 结构：`YYYY/MM/<UnixNano><ext>`，无前缀、无 day 层。
+// 与 route/question/page.go:uploadImage 的落盘 key 保持一致。
+var prodKeyRe = regexp.MustCompile(`^\d{4}/\d{2}/\d{16,19}\.jpg$`)
+
+func TestUploadKeyFormat(t *testing.T) {
+	key := uploadKey(".jpg")
+	if !prodKeyRe.MatchString(key) {
+		t.Fatalf("uploadKey(.jpg) = %q, 不符合生产口径 YYYY/MM/<UnixNano><ext>", key)
+	}
+}
+
+func TestExtFromContentType(t *testing.T) {
+	cases := map[string]string{
+		"image/jpeg": ".jpg",
+		"image/png":  ".png",
+		"image/gif":  ".gif",
+		"image/webp": ".webp",
+		"image/bmp":  "",
+	}
+	for ct, want := range cases {
+		if got := extFromContentType(ct); got != want {
+			t.Errorf("extFromContentType(%q) = %q, want %q", ct, got, want)
+		}
+	}
+}
+
+func TestOriginalExt(t *testing.T) {
+	cases := map[string]string{
+		"photo.JPG":  ".jpg",
+		"a.b.png":    ".png",
+		"noext":      "",
+		"tricky.PnG": ".png",
+	}
+	for name, want := range cases {
+		if got := originalExt(name); got != want {
+			t.Errorf("originalExt(%q) = %q, want %q", name, got, want)
+		}
+	}
+}
diff --git a/internal/storage/s3.go b/internal/storage/s3.go
index 7089889..45ab2e6 100644
--- a/internal/storage/s3.go
+++ b/internal/storage/s3.go
@@ -16,7 +16,6 @@ import (
 	"mime/multipart"
 	"os"
 	"os/exec"
-	"time"
 
 	"github.com/aws/aws-sdk-go-v2/aws"
 	"github.com/aws/aws-sdk-go-v2/service/s3"
@@ -24,7 +23,6 @@ import (
 	"github.com/disintegration/imaging"
 	"github.com/pkg/errors"
 	"github.com/sirupsen/logrus"
-	"github.com/thanhpk/randstr"
 	"github.com/wuhan005/gadget"
 
 	"github.com/tamakyi/TamaBox/internal/conf"
@@ -181,12 +179,9 @@ func uploadPictureToS3(file multipart.File, fileHeader *multipart.FileHeader, as
 		}
 	}
 
-	// 5. 生成存储路径（无后缀名）
-	now := time.Now()
-	year := now.Year()
-	month := int(now.Month())
-	day := now.Day()
-	key := fmt.Sprintf("%s%d/%02d/%02d/%s", PictureKeyPrefix, year, month, day, randstr.Hex(15))
+	// 5. 生成存储路径：`YYYY/MM/<UnixNano><ext>`（生产口径，无前缀、无 day 层）。
+	//    扩展名跟随最终落盘内容类型，保证 key 扩展名与 ContentType 严格一致。
+	key := uploadKey(extFromContentType(contentType))
 
 	// 6. 获取共享 S3 客户端
 	client, err := GetS3Client()
@@ -202,7 +197,7 @@ func uploadPictureToS3(file multipart.File, fileHeader *multipart.FileHeader, as
 			Body:          bytes.NewReader(uploadData),
 			ContentLength: aws.Int64(int64(len(uploadData))),
 			ContentType:   aws.String(contentType),
-			// key 为 picture/日期/随机hex，内容不可变 → 可安全启用 immutable 强缓存，
+			// key 含纳秒时间戳且内容不可变 → 可安全启用 immutable 强缓存，
 			// 避免每次页面加载都回源校验（曾导致头像/二维码 logo 每次重新下载）。
 			CacheControl: aws.String("public, max-age=31536000, immutable"),
 		})