When it comes to scanned images, a higher resolution isn't always better. Image resolution is measured as pixels per inch (ppi), and images scanned at a higher resolution store more information in the digital file.
You should relate ppi to lpi. Set the ppi at roughly 2 times the line screen (lpi). If your printer suggests that you use an lpi of somewhere between 70 and 106, you'll probably want to scan your photos with a ppi of between 150 and 175.
If you hire a service bureau to produce your camera-ready copy, you can scan images using the higher ppi. In any case, it's a good idea to discuss both scan resolution (ppi) as well as line screen (lpi) with your printing service ahead of time.