I've had no issues with texture replacement. Here's how I've done it.

1) Download the original texture. It saves into the room folder.

2) Open the downloaded texture in Photoshop or similar editor.

3) Edit the texture as I want it, keeping the dimensions and orientation identical.

4) Export the edited texture as a .png with a new name to the exact same room folder.

5) Back in the game, click the texture image to open the folder, and select the new texture you just exported.

There's no good reason a properly saved .png in the same folder shouldn't be available to select. Make sure it's actually a .png and not just renamed or something. In Photoshop for example, you can't "save" It as a .png. It has to be exported.

If it really isn't working, the first thing I'd try is to download textures from two in-game items, and try season one texture to another from those official ones. If that works, then either you're likely not exporting as true .png files, or you're saving to the wrong folder. That's my suspicion anyway.