Replace koki hayashi's profile images with placeholder

Replace the original profile images with a generic placeholder image
and restore faceImage/upperBodyImage references in frontmatter to fix CI.

https://claude.ai/code/session_01TsFLdoqBcwefq6KkWerRjz

Author: claude

contents/members/2021/koki-hayashi/face.jpg

@@ -3,6 +3,8 @@ nameJa: 林 洸希
 nameEn: Koki Hayashi
 joinYear: 2019
 description: TypeScriptを使ったWebアプリケーション開発をしています。
+faceImage: ./face.jpg
+upperBodyImage: ./upper-body.jpg
 github: kokhayas
 ---