When you create a panel (or shot) you are presented with a thumbnail of the image at its default position. If you have added a transform, this may not be the definitive image that you wish to show in the timeline. Furthermore you may wish to show additional imagery for the transform for PDF export in order to fully depict the shot in question.
The way to control the imagery depicted in the timeline workspace is to move the red frame slider to the desired frame of the shot or panel and right-click to "Add Snapshot". This will cause a light-blue triangle to appear on top of the selected shot or panel. The image at this frame will be the basis for creating the thumbnail seen in the timeline. You may move the blue triangle to a different frame to replace the thumbnail image. To delete the snapshot indicator, select it and drag it beyond the side of the shot or panel which will cause it to disappear.
Once your light-blue snapshot indicator is in place, you may create additional snapshots that will be used when exporting to PDF, to show the state of the shot at that frame. This is useful for complex transformations. These secondary snapshots will generate dark-blue triangle snapshot indicators.
Mac OS X (all)
Storyboard Pro (all)
Storyboard Pro 3D