If you want to convert your png file to srf file (garmin format), you can do it online or compile a C program to do it directly on your computer.

You just need a gcc compiler, the png library installed with include and the following sources in C language.

You will be able to convert png to srf and  srf to png.

http://myweb.cableone.net/eviltwin69/Garmin_srf_to_png_file_conversion.html

Enjoy !