software: making CMYK print files for free instead of $700

I'm helping a friend make postcards for SF Open Studios.

Rocket Postcards prints postcards from your source files. So take a nice picture; use a cheap or free image editing program like Photoshop Elements to resize it to 1275 x 1800 (4.25 by 6 inches at 300dpi) and tweak it; upload it to Rocket Postcards and order your postcards!

Not quite. Rocket Postcards requires your files be in CMYK mode. They have the grace to explain why:

All images must be in CMYK color mode. ... RGB images are not acceptable - if you normally design for web or multimedia, keep in mind that printed output uses a different color model (subtractive rather than additive) and corresponds to percentages of Cyan, Magenta, Yellow and Black inks rather than composite Red Green and Blue monitor light.

Fair enough, but your camera's images are RGB TIFF or JPEG files, how to get CMYK? Adobe wrote code to work in CMYK color space 17 years ago for Photoshop 2.0 in 1991. But unlike a free open source program, Adobe needs artificial reasons to make you pay $700 for Photoshop instead of $120 for the Photoshop Elements and Premier Elements combo. So there's no CMYK support in Photoshop Elements. Don't like it? Don't buy software.

As usual there are alternatives. The fine complicated free and open source ImageMagick software suite has a command-line tool to convert between image formats. The command line you want is

C:\my\docs>"C:\Program Files\ImageMagick-6.4.3-Q16\convert.exe"
    -colorspace CMYK -type ColorSeparation
    "original file" postcard_front.tif

To check, run imdisplay.exe and view it. Also run identify.exe -verbose:

C:\my\docs>"C:\Program Files\ImageMagick-6.4.3-Q16\identify.exe"
    -verbose postcard_front.tif

For some reason the CMYK image is in PixelsPerCentimeter, so size appears as

  Resolution: 118.11x118.11
  Print size: 15.24x10.795
  Units: PixelsPerCentimeter

Setting -units PixelsPerInch doesn't work, I wasn't sure how to change this without resampling. Programs agree the size is 4.25 by 6 inches. Hooray for free software!