You don't want monochrome or black and white. You must create a 24-bit COLOR PNG. Even if the contents are black-and-white, the image must be stored in color. It must be exactly 256 x 256. And finally, if you want the image to be "cut out" on the crystal instead of a solid box, you must handle the transparency.
The Photoshop instructions are here:
https://www.feldoncentral.com/Sachs/malogo/guide/
Once you are done with this, you drag-and-drop the PNG file to your C:\Program Files\SereneScreen\Marine Aquarium Time 2\ folder.
If you want to use the "Import Logo" button in MAT, that is a whole different process. You would need to create a black-and-white BMP file. The graphic quality when you do this is very limited!