diff --git a/public/scripts/extensions/shared.js b/public/scripts/extensions/shared.js
index 37dbec96a..26164d8d0 100644
--- a/public/scripts/extensions/shared.js
+++ b/public/scripts/extensions/shared.js
@@ -23,7 +23,7 @@ export async function getMultimodalCaption(base64Img, prompt) {
     const compressionLimit = 2 * 1024 * 1024;
     if (extension_settings.caption.multimodal_api === 'openrouter' && base64Bytes > compressionLimit) {
         const maxSide = 1024;
-        base64Img = await createThumbnail(base64Img, maxSide, maxSide);
+        base64Img = await createThumbnail(base64Img, maxSide, maxSide, 'image/jpeg');
     }
 
     const apiResult = await fetch('/api/openai/caption-image', {
diff --git a/public/scripts/utils.js b/public/scripts/utils.js
index d8b99956f..571583240 100644
--- a/public/scripts/utils.js
+++ b/public/scripts/utils.js
@@ -973,9 +973,10 @@ export function loadFileToDocument(url, type) {
  * @param {string} dataUrl The data URL encoded data of the image.
  * @param {number} maxWidth The maximum width of the thumbnail.
  * @param {number} maxHeight The maximum height of the thumbnail.
+ * @param {string} [type='image/jpeg'] The type of the thumbnail.
  * @returns {Promise<string>} A promise that resolves to the thumbnail data URL.
  */
-export function createThumbnail(dataUrl, maxWidth, maxHeight) {
+export function createThumbnail(dataUrl, maxWidth, maxHeight, type = 'image/jpeg') {
     return new Promise((resolve, reject) => {
         const img = new Image();
         img.src = dataUrl;
@@ -1000,7 +1001,7 @@ export function createThumbnail(dataUrl, maxWidth, maxHeight) {
             ctx.drawImage(img, 0, 0, thumbnailWidth, thumbnailHeight);
 
             // Convert the canvas to a data URL and resolve the promise
-            const thumbnailDataUrl = canvas.toDataURL('image/jpeg');
+            const thumbnailDataUrl = canvas.toDataURL(type);
             resolve(thumbnailDataUrl);
         };