The DCI specifications demand that for a 2k DCP, which has a container size of 2048x1080, one or both sides of this container must be completely filled and that the container size must not be exceeded.
However, commonly 2k DCPs are eigther Flat (1.85:1) 1998x1080 or Scope (2.39:1) 2048x858.
If your input material does not meet one of these resolutions, you can individually configure for each picute track file how it shall be resized, using a combination of pillar-boxing (black bars left and right), letter-boxing (black bars top and bottom), scaling and/or cropping.
To do that, just follow these steps:
1) Add a new Picture track file
2) Right-click on the track file to show the context menu
3) Browse to Letterboxing and select one of the entries
To see, what resolution is used, refer to the Metadata Inspectorwindow´s Target Resolution line.
Optionally, you can also scale your centent
Rather than repeating these steps for all track files over and over, you can also predefine a default pillar-/letterboxing setting in the global Preferences.
Just browse to the "New Track Defaults" tab and select the default letterboxing for new track files.