I have been working as a graphic designer for over 35 years. What I prefer is to convert all my rgb jpgs to CMYK pdfs, then link them into my InDesin doc. The printer will convert it automatically during the RIP process, but if you want to control the outcome of your photos, I recommend converting them yourself and adjusting as needed. I generally like to adjust the photos using levels. (Lighten overall and then bring back the black level).
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