"A photo that I took was used for an advertisement on the back cover of a magazine."
- "A photo that I took was used for an advertisement on the back cover of a magazine."
"a photo that I took" 「わたしの撮った写真」"a photo I took" も言えます。
"was used for ~" 「〜として使われた」・「〜として載りました」
"an advertisement" 「広告」"an ad" も言えます。
"on the back cover" 「裏表紙に」
"of a magazine" 「雑誌の」