Printing PNG from command line

and printing other files from the command line  

The easiest method is if you are printing one of the following file types:
Text, PDF, PostScript, PNG, JPEG, BMP, or GIF.

Simply run:
lpr -o fit-to-page filename.png

Or possibly:
lpt -o landscape -o fit-to-page filename.png

If you need to print a different file type you may be able to use: convert

For example:
convert page1.png ps:- | lpr -o fit-to-page

This will read page1.png, convert it to PostScript and write the output to stdout.

For jpg output you could use:
convert page1.png jpeg:- | lpr -o fit-to-page