My experience with postcards and flyers has always been that jpegs give the most predictable result. Print houses field lots of them, and the constant feedback keeps them in the ballpark. For newspaper articles I have submitted both jpegs and CMYK conversions, and the jpegs without exception have looked better in print. If possible pick images without saturated or dark blues, keep important detail in the mid tonal range, emphasize large scale shapes if possible, and go easy on the saturation overall.