Can't extend image duration in Final Cut Pro X
Try this

At the end of your main timeline add a gap Option-W. 
Drag and drop the still picture behind the gap into the main timeline. 
Change the duration as needed. 
Move the clip where you want it. 

Your behaviour you described isn’t normal, it sounds like you do everything correct. Above is to eliminate any timeline issues from stacking and try to sort out the problem.