On a machine I still have Final Cut 6.0.6 installed I keep running into a bug with Still-Freeze Durations.
The duration Final Cut assigns an imported still image is determined by the Editing tab of the User Preferences (Option + Q).
However, for me in Final Cut Pro (and if I'm recalling correctly, versions after 5.0) the Still portion of Still/Freeze Duration doesn't exactly work.
In my experience when you import a still image FCP will set it's duration to exactly one second less than what's set in the Editing prefs. The Freeze duration will be correct however, meaning that if you have a clip loaded into the Viewer and hit Shift + N to make a freeze frame it'll be whatever the Editing tab's Still/Freeze Duration is set to.
But, if you say, drag a photo into the bin it'll be one second shorter.
On the left is the bin, on the right is the preference. For me, it's consistent.