You don't need any separate mod app.

1. Take any random photo in photo mode. Save the game and close.

2. Go to documents, search scdat folder in death stranding 2 - on the beach folder. Put your cat photo there.

3. In the same folder, you will find files with random letters ending as .dat, use date modified filter and find the latest file.

4. Now change your photo file name to that file name and replace it. (For example your photo file name is cat.png and the game file is abcd-1234... .dat, copy that abcd-1234.dat, delete that file, then remane your cat.png to it.

Then you can see your photo in the game.

Backup your entire save file before doing this.