Appears I’m coming late to this party, and you have probably moved on, but when I want to freeze a frame I typically use ‘frame hold’. To do this, I trim the clip with the image I want so that the image is the first frame of the clip. Then I right click on the clip and choose ‘Frame Hold’. On that dialog box click the ‘Hold On’ box and make sure the drop down says ‘In Point’. If needed also click the ‘Hold Filters’ and ‘Deinterlace’ check boxes. You can size the clip to the amount of time you want by extending its end (don’t extend from the front of the clip, as that will change the frame at the In Point).
Since it sounds like you want a pseudo photo montage, you may want to try the ‘Posterize Time’ video effect.
Pauley