I have been creating artworks on this topic for several years.
My artworks have been focused on this idea for the past several years.
I've been doing photographic works on this theme for some years.
「テーマ」はドイツ語の「Thema」からの外来語です。英語では「theme」という言葉があって意味はだいたい同じですが、「topic」や「idea」でも使えます。
軸に = focused on, revolving around, on, around, based on
仕事の写真 = photos taken for work (work = 仕事)
アートとしての写真 = works, artworks, art photography (数えられない名詞)
アメリカ合衆国 
