Maya supports several image formats for stamps. While you can use 8-bit images, you can achieve better results with images that are 16 or 32-bit depth.
To create stamps with an alpha channel (RGBA) and have 16 bits per channel as their bit depth, you can use an editing application such as Adobe Photoshop.
To use Photoshop for stamps
(Creates an alpha channel for the image.)
(Ensures the file is not tagged with an existing color profile.)
A new blank image appears in the work area. A checkered background on the canvas indicates the presence of the alpha channel.
The Height and Width for a stamp image can range between 256 and 512 pixels, and must be square in proportion.