There are no workflow details or screenshots showing the problems you are having so I have no idea what kind of user error there is in the procedure. I don't have time to watch the tutorial and try and guess what you are doing wrong but I think it has something to do with not having a good understanding of the basic UI, workflow and animation techniques used in After Effects. I did watch the first part of the tutorial and it seems to be a fair rather than an excellent tutorial on using AE. If you are new to AE and judging from the first minute or so of the tutorial you should probably spend an hour or two learning about the basics of the AE user interface and the techniques used to add and replace footage. You can start here: Basic AE
If you are experienced and are just stuck then a screenshot of your comp with the modified properties of all layers involved and the Project Panel showing would help. Just press the U key twice.
If you change one image and it changes them all you are probably working at the wrong level in a series of nested comps (sometimes called pre-comps). Check out the Flow Chart to see where the assets in the supplied template are actually located.
You try to select all the files you want from the top left menu and drag them into timeline. You confirm them on timeline and then you adjust the starting point and the ending point for play.