書き出し時にサムネイルを設定することは可能でしょうか？

After Effects (en) から After Effectsフォーラム (jp) に移動しました。

Long and short: No. Most video formats don't support arbitrary poster frames. Any such stuff is typically merely dependent on viewers or operating systems extracting the first frame (or somewhere in teh middle) and generating a thumbnail for it. You have no control over these things.

Mylenium