What were you trying to achieve by creating this picture?
What was your motivation for painting that picture?
Mkさん
ご質問どうもありがとうございます。
様々な表現があると思いますが、いくつか自然な表現を紹介します。
1. What was your goal in painting that picture?
この絵を描くゴールは何でしたか?
2. What were you trying to achieve by creating this picture?
この絵を描くことで、何を目指していたんですか?
3. What was your motivation for painting that picture?
この絵を描くの動機付けは何でしたか?
P.S. 絵の作者本人に聞くのでない場合は、下記の言い方でいい思います。
1#. I'd really like to know what his goal was in painting that picture...
2#. I'd really like to know what he was trying to achieve by creating this picture...
3#. I'd really like to know what has been his motivation for painting that picture...
お好みに合わせて使い分けてみてください。
ご参考にしていただければ幸いです。