It’s not necessary to flatten Photoshop layered files before placing them in an InDesign page. In fact, unless a Photoshop file has reached an unwieldy size, there are compelling reasons not to flatten it because of the flexibility and editability you sacrifice when you flatten. Maintaining live layers means simplified housekeeping because there’s no need to keep a separate working image file. But there are some situations that present challenges.
Be sure to save Photoshop .PSD files with Maximum Compatibility turned on, both for compatibility with older (and future) versions of Photoshop, and for better performance in InDesign.
Photoshop, Illustrator, and InDesign make it easy to add drop ...