Converting photos to CMYK

Photos cannot do a CMYK conversion. The normal tool for this is Photoshop, but obviously that is out here.

It looks like GIMP can do it.

CMYK conversion is pretty esoteric, I have no idea why a competition would require it.

GraphicConverter can convert to CMYK. I use its demo version for the rare occasions I need to have a CMYK file.

The winning images will be printed in a magazine; I guess they don't want to do the conversion themselves, which is understandable if they are dealing with a large number of images.

Nah. That's the way all better (bigger) print shops do it now. Much of the time, they don't even convert the images beforehand. Anything that comes in as RGB goes into InDesign or Quark XPress as is. The conversion is handled at the RIP so color is converted to a single, consistent CMYK inks setup, and is all automatic.

It looks like Affinity can do it.

Yes, that one can, too.

CMYK conversion is pretty esoteric, I have no idea why a competition would require it.

That is a very odd requirement. I would imagine it's because they're planning on printing a book of winning images to sell later. But the printing company they hire to do that can do the RGB to CMYK translation. There's no good reason for the photographers to be required to send it in that way.

CMYK also isn't straight forward. There are many different ink setups you can use that will give you as many equally different conversion results. Even your computer's setup makes a very big difference. You absolutely, no quibbling, must use a computer correctly calibrated and profiled to 5000K, 1.8 gamma. This is the only way you'll get a balanced, neutral gray conversion from RGB, Grayscale or Lab without having to fidget with the image afterwards. There's no (competent) print shop that doesn't use these settings.