MapTiler Desktop allows users creating own watermark. This functionality is available in MapTiler Desktop PRO version.
Creating custom watermark is available in Advanced options, which appears after georeferencing an image.
Watermark can be created either from an image or text, a combination of both is not possible.
Image as a watermark
To create a watermark from an image, load the file in PNG format by clicking on the image file link. The watermark is randomly distributed on each of the 256x256 px tiles.
If you want to have your watermark aligned regularly in a grid, create 256x256 px image with watermark inside.
256px is the max limit for a picture in any dimension.
Text as a watermark
Click on the text link and write the desired text.
MapTiler currently supports only in-line notation.
Move the opacity slider to define transparency of the watermark where 0% means fully transparent and 100% opaque image/text.