This wasn't there when I took a picture of my dog a month ago.
I don't remember seeing this when I took a photo of my dog a month ago.
ーThis wasn't there when I took a picture of my dog a month ago.
「1ヶ月前に犬の写真を撮った時には写ってなかった」
this wasn't there で「これはそこになかった」=「これは写ってなかった」と言えます。
to take a picture of ... で「…の写真を撮る」
a month ago で「1ヶ月前」
ーI don't remember seeing this when I took a photo of my dog a month ago.
「1ヶ月前に犬の写真を撮った時、こんなのを見た記憶がない」
I don't remember seeing ...「…を見た覚えがない・記憶がない」を使っても表現できるかと思います。
ご参考まで!