Place your skin(s) ".osk" into the /input/ folder 
NOTE: Rename skin file if it has "special" characters for compatibility. 
Run "Run.sh" with Git for Windows 
Wait until completion all should go well if you've properly installed the right programs. 
You will find HD/SD, HD, and SD in /output/skinname/ 

Note: There is only light compression on JPG and MP3, should not be noticable. Bit-depth of png's are reduced, again, not noticable. 