I know I can extend the duration of a background image if I click and drag the end of the image, but it’s hard to do so if the desired final length of the image needs to be 6 hours.
I wonder if there is a quicker way to have the background image extended from the beginning to the end of the 6 hours?
I’m not sure, it always works for me, however I must add that I have never made a still that long.
Perhaps SC sees it as too long and reverts back.
Can you try with something a bit shorter, maybe 3 hours?
If that works then just copy and paste and you have your 6 hours total.
You can use the Properties > Duration when the image is in Source before adding it to the timeline. When it is already in the timeline, you cannot currently and must drag edges. You can do many things to anything while it is in Source before adding it to the Timeline: trim, Properties, Filters, Keyframes. This is a fundamental concept.