You can use cat() instead of writeLines():
writeLines("Image not supplied.\nUsage: ./program F=filename")
Image not supplied.
Usage: ./program F=filename
You don't have to remember to append a "\n" to the string passed to cat() to get a newline after your message.
E.g. Compare the same above output:
cat("Image not supplied.\nUsage: ./program F=filename")
Image not supplied.
Usage: ./program F=filename>
and
cat("Image not supplied.\nUsage: ./program F=filename","\n")
Image not supplied.
Usage: ./program F=filename