Don't see that here. Placing video, an image or audio clip on a separate (overlay) track should not affect the main track. A track can be locked via a right-click at its far left, but that shouldn't be necessary.
Elaborate a bit on what you mean by placing it "in line with the video track." Include a screen capture if possible. Here's how...
Upload the capture file 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."