Difficulty with multiple formats

I am cutting a 21st Birthday video which includes archival footage, stills, current video, etc. Everytime I change anything I lose the formatting on each photo and have to redo it. How can I lock in the sizing and positioning so this does not revert to the original inputted format.

Set the Sequence settings once, and never touch them again. Resize each individual media file as needed, in the Viewer. Or am I missing something here?