By default, most digital cameras save RGB or sRGB images. This offers the widest color range for monitor display and inkjet reproduction of individual photos. Rich quadtone grayscale printing of black and white images can also be output from RGB images. When an offset press is the target device, photos should be converted to grayscale.
Offset printing, with its inherent CMYK limitation is still most economical for large print quantities. Therefore, when many images will be repurposed for multiple products -- say a single large poster, 50 color laser prints and several thousand brochures -- it is often more efficient to prepare art with CMYK images.
Converting images from one color space to another can be accomplished with Adobe Acrobat but is preferably done with an image editor like Photoshop.