If all clips are on the same track then preview will display only the clip where the scrubber (red line) has been placed.
If clips are stacked on different tracks, one above the other, higher track/s take precedence.
So, if an image on track 2 is small and the one on track 1 is large, then the track 1 image will be seen in the background.
Picture-in-picture. http://help.nchsoftware.com/help/en/videopad/win64bitpaid/tasks_watermarkspip.html
Individual tracks can be hidden temporarily. Click the 'eye' 
If this doesn't describe what you are seeing, please upload a screenshot and link it here...
Upload to a free server, such as Google Drive or MS OneDrive, get a shared (public) link, copy the link and paste it here or to me in a Private Message via the mail envelope in the top-right corner of this forum. When using Google Drive, if necessary change "restricted" to "anyone with link can view."