Using r.thin in GRASS

I am new to GRASS GIS. I need to run my program with the r.thin command, and I will not get any results as I proceed according to the program. My map is a png image in 2 bits.

How do I execute this command correctly?

My input image in "png" format is as follows:

And my output image after executing the "r.thin" command is as follows:

As it is clear, no thinning has been done and only the color of the image has changed.