Can someone please ELI5? When I hear live images, I think of iOS videos that go along with pictures you take
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A reproducable build means you can get the same source code and compile it, and it will be identical to the published image. This is important because otherwise you don't know if the published image actually used some other source code. If it used some other source code, the published image might have a backdoor, or something that you can't find by reading the source code.
Also we have had live images of various OSes for many decades. I seem to recall that we used to load DOS from floppy disks.