fix(video.py): remove unnecessary prompt for generating thumbnail

fix(video.py): assign url and id properties after uploading video
fix(montage.py): fix unsplash_url to use source.unsplash.com instead of api.unsplash.com
fix(montage.py): fix image download logic in prepare function
fix(marp.md): add missing newline at end of file
fix(wiki_downloader.py): fix image download logic in download_image function

prompts/marp.md

@@ -5,4 +5,29 @@ class:
     - lead
     - invert
 backgroundImage: url(https://images.unsplash.com/photo-1651604454911-fdfb0edde727)
----
+---
+<style>
+    section {
+        display: flex;
+        justify-content: center;
+        align-items: center;
+    }
+
+    .image-container {
+        width: 90%;
+        max-height: 90%;
+        display: flex;
+        justify-content: center;
+        align-items: center;
+    }
+
+    .image-container img {
+        object-fit: contain;
+        width: 100%;
+        height: 100%;
+    }
+</style>
+
+<div class="image-container">
+    <img src="[imagesrc]"/>
+</div>