The picture you drew is just how I imagined my birthday party would be!
This is just what I thought my birthday party would look like!
ーThe picture you drew is just how I imagined my birthday party would be!
「あなたが描いてくれた絵はまさに私が想像した誕生日パーティーです!」
... is just how I imagined ~ で「…はまさに私がイメージした通りの〜です」
ーThis is just what I thought my birthday party would look like!
「これはまさに私が思った通りの誕生日パーティーです!」
This is just what I thought ... で「これはまさに私が思った通りの…です」
ご参考まで!
This drawing is exactly what I imagined for my birthday party!
This drawing is exactly what I imagined for my birthday party!
This is just how I pictured my birthday party.
「この絵はまさに私がイメージした誕生日パーティー!」は、英語だと exactly what I imagined(まさに私が想像していた通り)や just how I pictured(私が頭の中で思い描いていた通り)を使うと、とても自然です。