duration in images

I am editing video shot in front of a green screen. I want to add still images as new background such as the an image of a book cover- written by the person speaking. I cannot find a way of adjusting the duration of this book cover image. Under "help" the duration for the mag. timeline works just fine but nothing seems to work to adjust/lengthen a still image in the background (under timeline). pls help.
Thx Sabine

Highlight the asset in a Timeline, Control-D will show its duration in the Dashboard. Simply type in a new duration and press Enter.

For example, if I want an asset to be 4 seconds and 15 frames, I can simply type in 415 and hit enter. FCP automatically counts from the right and places the colon in the correct place.