How do load in a source frame/video?

In the “Preview Background” dropdown, I have the option to either view the effect over a “Source Frame” or a “Source Video”. How do I load in a source frame/video to put the effect over and match up to make sure it works with my video I’m doing the effect over?Screen Shot 2021-10-19 at 1.09.56 PM

You can’t load video in the standalone version.