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I already have one photo that is in jpeg format that is 300dpi, but I did not create that. I scanned in a photo at 300dpi as a tif file, and would like to convert it to jpeg while preserving its 300dpi resolution.
If the printer's designer is going to do some of the final composition, he will be able to use your tiff and your jpeg.
Either way it seems to me you don't need to convert your tiff to jpeg.
But I'm not a designer (or a printer), so there may be something I've missed.
BTW, when I try switching it to grayscale, it says, "Changing modes will affect layer compositing. Flatten image before mode change?" I assume this is OK to do in this situation. I obviously won't do it until I am completely finished.
Sometimes grayscale conversion doesn't give a very good looking image. Another approach you might try after flattening the layers is to go to L*a*b color space, go into Channels and then select the L channel (Lightness). Select All, Copy and then return to RGB and paste the copied channel onto your image and flatten again.
The "L" channel in L*a*b color space often preserves subtle shades of gray detail and a better black to white distribution than a straightforward grayscale conversion (or a "Desaturate, which also removes all color.)
Additionally, JPEGS sometimes have issues with particular RIP processes, so TIFFS are the preferred filetype for professional image manipulation.
Lastly, "Changing modes will affect layer compositing. Flatten image before mode change?" is a safety mechanism that Photoshop has regarding color shifting. It's just a warning that color *may* be altered. It's often alright to work with the file even after the mode switch WITHOUT flattening the file. Just keep in mind that a shift has occurred and be careful to watch for color change.