From the changelog of 1.3.4

Add the ability to use custom splash screen background images placed in a resourcepack under "assets/minecraft/puzzle/splash_background.png"

What about supporting the OptiFine/official filepath? That being assets/minecraft/textures/gui/title/mojangstudios.png.

Nevermind, it did support logo swap since 1.17 and you just added a feature to replace color with bg... my bad 😬

https://github.com/PuzzleMC/Puzzle/blame/795759b4e559173a4c20379ea62baa7ac6434185/puzzle-splashscreen/src/main/java/net/puzzlemc/splashscreen/PuzzleSplashScreen.java#L36