Go to your /rez2 folder in your crossfire directory and take out the last .rez file (Na is RF159.rez , EU is RF144.rez).
Then extract the .rez file to a folder, inside the folder create a new folder named MARK (case-sensitive) and add your kill-marks.
Use a application to make the folder into a .rez file with the same name (names are shown above for the versions) and place it in your
/rez2 folder