Can someone who understands coding help me out? I've got like 10 tabs open, read through all instructions for creating custom cave displays and I'm still struggling hard-core and need some assistance badly

If I reference anything here about codes I'm referring to this guide I attempted to follow (but don't understand cause I can't figure out how to do it)

https://www.lioden.com/topic.php?id=304429347355

So All I'm trying to do is just change the images for my Caves on my page.

I've made a Dropbox account and copied the code but I'm not sure if I should put the whole code or just the "CAVE" portion of the code - and if I just use the cave portion do I need to keep the page credit section? Like should I copy the credit part, paste it in, then paste multiple CAVE code portions following it and just upload that? Or if I paste the whole template code and just change the CAVE customizing part, do all the other sections, like header and boxes for instance, just appear but with like "CODE HERE" text written cause I didn't change anything for the rest of the code spots you're suppose to fill with something?

Also another thing confusing me is how to create and upload your own template to begin with. On Dropbox has options to create document or a web link but also options to upload a file. Are you suppose to paste the codes you want to use into a Notepad note and somehow save it and upload that as a file? Or create a document on Dropbox? Idk I feel like I'm circling around the answers but it just doesn't make sense to me at all All I wanted to do was have cool cave backgrounds

Also for images! I've followed the links to like lioden backgrounds and the other stock image sites. So you can find a link to an image by going "open image in new tab" and get the link that way. But the pictures I want to use have to be resized to fit the image size requirements for each field.
So I download the image and resize it but now I don't have a link to it, I only have the downloaded & resized image. Do I paste the image code in there somehow or do I have to then upload the resized image somewhere in order to use it?

And also, if I have a custom den display already in use but it doesnt affect the cave displays at all, can I still do all this and just change the cave images? Or will it mess up my custom den background? Or are they unrelated and can be changed around separately?

I am so very lost
I'd love any advice or help anyone has (if it could be in most simple terms that would help too! I'm not a coder I don't understand many terms of the trade)
Thank you so much to anyone who responds!
You're saving me, I'm drowning in codes haha