I want to print an image with a size of 300x300 pixels, I've used this command
$ lpr -P Myprinter imagefile.png
The image was printed fit-to-page by default.
How can I print this image without scaling?
You have to configure the ppi of your printer using the ppi flag:
The ppi=value value is a number from 1 to 1200 specifying the resolution of the image in pixels per inch. An image that is 3000x2400 pixels will print 10x8 inches at 300 pixels per inch, for example. If the specified resolution makes the image larger than the page, multiple pages will be printed to satisfy the request.
more info: LPR Command-Line Printing and Options