diff --git a/src/lib/backpack/costume-payload.js b/src/lib/backpack/costume-payload.js
new file mode 100644
index 000000000..a376c1111
--- /dev/null
+++ b/src/lib/backpack/costume-payload.js
@@ -0,0 +1,33 @@
+import jpegThumbnail from './jpeg-thumbnail';
+import storage from '../storage';
+
+const costumePayload = costume => {
+    // TODO is it ok to base64 encode SVGs? What about unicode text inside them?
+    const assetDataUrl = storage.get(costume.assetId).encodeDataURI();
+    const assetDataFormat = costume.dataFormat;
+    const payload = {
+        type: 'costume',
+        name: costume.name,
+        // Params to be filled in below
+        mime: '',
+        body: '',
+        thumbnail: ''
+    };
+
+    switch (assetDataFormat) {
+    case 'svg':
+        payload.mime = 'image/svg+xml';
+        payload.body = assetDataUrl.replace('data:image/svg+xml;base64,', '');
+        break;
+    default:
+        alert(`Cannot serialize for format: ${assetDataFormat}`); // eslint-disable-line
+    }
+
+    // TODO we may be able to optimize by not actually generating thumbnails...
+    return jpegThumbnail(assetDataUrl).then(thumbnail => {
+        payload.thumbnail = thumbnail.replace('data:image/jpeg;base64,', '');
+        return payload;
+    });
+};
+
+export default costumePayload;
diff --git a/src/lib/backpack/jpeg-thumbnail.js b/src/lib/backpack/jpeg-thumbnail.js
new file mode 100644
index 000000000..623daf3be
--- /dev/null
+++ b/src/lib/backpack/jpeg-thumbnail.js
@@ -0,0 +1,20 @@
+const jpegThumbnail = dataUrl => new Promise(resolve => {
+    const image = new Image();
+    image.onload = () => {
+        const canvas = document.createElement('canvas');
+        const ctx = canvas.getContext('2d');
+        // TODO we may want to draw to a smaller, fixed size to optimize file size
+        canvas.width = image.width;
+        canvas.height = image.height;
+
+        ctx.fillStyle = 'white'; // Create white background, since jpeg doesn't have transparency
+        ctx.fillRect(0, 0, canvas.width, canvas.height);
+
+        ctx.drawImage(image, 0, 0);
+        // TODO we can play with the `quality` option here to optimize file size
+        resolve(canvas.toDataURL('image/jpeg', 0.92 /* quality */)); // Default quality is 0.92
+    };
+    image.src = dataUrl;
+});
+
+export default jpegThumbnail;
diff --git a/src/lib/backpack/sound-payload.js b/src/lib/backpack/sound-payload.js
new file mode 100644
index 000000000..e5e1c992a
--- /dev/null
+++ b/src/lib/backpack/sound-payload.js
@@ -0,0 +1,31 @@
+import soundThumbnail from './sound-thumbnail';
+import storage from '../storage';
+
+const soundPayload = sound => {
+    const assetDataUrl = storage.get(sound.assetId).encodeDataURI();
+    const assetDataFormat = sound.dataFormat;
+    const payload = {
+        type: 'sound',
+        name: sound.name,
+        // Params to be filled in below
+        mime: '',
+        body: '',
+        thumbnail: ''
+    };
+
+    switch (assetDataFormat) {
+    case 'wav':
+        payload.mime = 'audio/x-wav';
+        payload.body = assetDataUrl.replace('data:audio/x-wav;base64,', '');
+        break;
+    default:
+        alert(`Cannot serialize for format: ${assetDataFormat}`); // eslint-disable-line
+    }
+
+    payload.thumbnail = soundThumbnail;
+
+    // Return a promise to make it consistent with other payload constructors like costume-payload
+    return new Promise(resolve => resolve(payload));
+};
+
+export default soundPayload;
diff --git a/src/lib/backpack/sound-thumbnail.js b/src/lib/backpack/sound-thumbnail.js
new file mode 100644
index 000000000..f965cc8e1
--- /dev/null
+++ b/src/lib/backpack/sound-thumbnail.js
@@ -0,0 +1,3 @@
+// image/jpeg base64 encoded sound thumbnail image
+// eslint-disable-next-line max-len
+export default '/9j/4AAQSkZJRgABAgAAZABkAAD/7AARRHVja3kAAQAEAAAAPAAA/+4AJkFkb2JlAGTAAAAAAQMAFQQDBgoNAAACZgAAAwoAAAPwAAAE/P/bAIQABgQEBAUEBgUFBgkGBQYJCwgGBggLDAoKCwoKDBAMDAwMDAwQDA4PEA8ODBMTFBQTExwbGxscHx8fHx8fHx8fHwEHBwcNDA0YEBAYGhURFRofHx8fHx8fHx8fHx8fHx8fHx8fHx8fHx8fHx8fHx8fHx8fHx8fHx8fHx8fHx8fHx8f/8IAEQgAQABAAwERAAIRAQMRAf/EALAAAQADAQEBAAAAAAAAAAAAAAAEBQcGAgMBAQEBAQAAAAAAAAAAAAAAAAABAgMQAAEEAgICAwAAAAAAAAAAAAMBAgQFIBEQQDASYCQGEQACAQICBggHAAAAAAAAAAABAgMAESFBEDFRcRIEIDBhoSIyQhNAgZGxwWJTEgEAAAAAAAAAAAAAAAAAAABgEwEAAQIGAgIDAQAAAAAAAAABEQAhECAxQVFhQIEwcZGhsfD/2gAMAwEAAhEDEQAAAdUAAAAAIVk2UACFZz2pwvSa1x19ordLrNGVdsVNDvedjlXppXLQy7ril0He87GKzTSuWgKazOOudU5a9RWaXuaAAAAAAAB//9oACAEBAAEFAvGWbFE/E82KBZt8JGuc5zoa/SOdogV1myZzaKq2HE8b30lAX3ZW7BbcW4iMncTyuZSfnxeqV2z2/M2rjyWHjGAWMPUM8dpY9bWJD6X/2gAIAQIAAQUC6Ws0wXBMFx3hvpf/2gAIAQMAAQUC+Af/2gAIAQICBj8CAf/aAAgBAwIGPwIB/9oACAEBAQY/Aur9uSUK5y6VpZApOVFeV8Tn15D60WY3J1moCf5r9qabWFF8KYcPA6+m98O7TNfbphKnBFjLDaLVNyz4pa4G/A0I/wBmjOmQsMHN1OmEAedUUn5XqbmWwS3CD3mhJtZpD0DgElycfmvakXxZdu6oo5BqRQyndTw+UMLYZUzFuN2wv2fBf//aAAgBAQMBPyH4w+6y888e8wK/Yd2OYJamasJkfQXL6ikQPlLqtEOQX1+tJ6/x3VtNruFuMYphTp9AYwnOihP0s1zyk4sfdqu5Y7GJP6GLvTiVE2cZP3XRH9ointmkOrvq1QQW6QXf65Is2rxfoalPUHFc4PKtckUTcIiUEb+UburVx0sIgbb6+F//2gAIAQIDAT8h+OM0UYHBIx0YmuF0zDXC6ZCk04MvC//aAAgBAwMBPyHwpzuQyOQyxkjwv//aAAwDAQACEQMRAAAQkkkkkhkklIrEhkPEnkPEk6Pkkkkkkkkk/9oACAEBAwE/EPjj8Ql0GhYJP6UIgjI3EylYiQdlZgdxTFjQf2wHCn2aU4Yj6glVdVokAKqA31aJdngG0sDpd3oXX8WyOINm3HOLiiIlvAAejFrIbqFisawfSkBz6gD8RlZ90xKenkOY72Ng0hMc/A2cVHA50nk/5TSE7IQn4yF33UxR3IRz6DFBESRsjSwYJBTwiTd34oYrYAQuoCx6/tIGo6gKAkslIWGBBsIIQO1OlGBFumAquhfo8L//2gAIAQIDAT8Q+MTmE6Vy4amgljJtDF30N6vxK2JvpbVbkIUBJpXoYZ8Of//aAAgBAwMBPxD45zLTgNKWhnHVi6YDXHVi6YDXIypKKSiHhf/Z';