I needed to convert an image to PNG format so I have a transparent background. Unfortunately, CMYK isn't supported by the PNG format. I just wanted an image with a clear background so I can have that image alone in my design. I chose CMYK because I'm currently designing a book to print. Help.
Because PNG is a lossless raster image format developed for the web, and ultimately for display screens which are RGB.
However the format was never intended or designed to be a print format, and therefore doesn't need to support CMYK colour, and so it's entirely the wrong format to use for CMYK printing - i.e printing where separations are required for making CMYK printing plates/screens, such as in lithography, or screen printing, etc.
The de facto standard raster image format for CMYK printing is TIFF.
Note: Even though TIFF does support transparency, there's rarely a need for that when using it for CMYK printing. Everything that is white in a CMYK image is non-printing, because white ink is not used in that printing process.
Broadly speaking there are two schools of thought on file-format design.
One is that you should have lots of options to give lots of different users what they want. The problem with this approach is that compatibility becomes a problem. Just because a program advertises support for tiff files doesn't mean it can open your particular tiff file.
The other is that you should keep the number of options for the core functionality down, so that a relatively simple reader can read all files in the format. This is the approach that PNG took. It was designed as an image-format for the web in the late 90s and the set of options for the core functionality reflected that. The color options were RGB (with an optional alpha channel), palleted with a RGB based pallette and greyscale (trivially converted to RGB).
Yes PNG does have options for specifying specific RGB colorspaces for those who are picky about their colors, but simple decoders that ignore those aren't really any worse off than if the color space information had never been stored at all, so it doesn't break the basic compatibility.
The question of CMYK is actually explicitly addressed in RFC 2083
There is no support for CMYK or other unusual color spaces. Again, this is in the name of promoting portability. CMYK, in particular, is far too device-dependent to be useful as a portable image representation.
Look at it this way: PNG was developed as a replacement for GIF and so generally to be used in digital work, which means RGB (screens use RGB). And yes PNG is limited to RGB. While CMYK is a print-specific model available in JPG**, TIFF, PSD and some other formats. Read:
** JPGs cannot have transparent backgrounds