NextGen Templates?

I get the feeling perhaps this topic has been run into the ground.
If not, great! I hope someone can provide some assistance.

I’ve put together a wordpress site (a little bit of a hack job considering my inexperience with php and wordpress). I was introduced to Nextgen and would like to use it’s template feature. I figure this is the closest thing to having a different thumbnail image to the one that it’s linked to… like the basic html here:
<a href="image"><img src="thumbnail"></a>

I’ve been going around in circles and the closest thing to instructions is a quick run-through by a guy who knows what he’s doing- before I paste that though, this is my page-leap process:
https://nextgen-gallery.com/templates/example-1/ -> https://webdesignerwall.com/tutorials/css-decorative-gallery -> https://nextgen-gallery.com/templates/
“It is possible. I know the NextGen Gallery site mentions but doesn’t have any description on how to do it. You have to download the static HTML examples from this site then follow the directions to create the template file for NextGen gallery in your site’s theme folder (https://nextgen-gallery.com/templates/). This is a PHP file. You will have to copy the elements over from the example you wish to use and do some code editing. I created gallery-template.php file in the directory mentioned above, I also put the image file and css in that folder too. Don’t forget to call the gallery and include the template you wish to use with the short code: [ nggallery id=1 template=sample1 ]“

So I set it up as he says, I got a “nggallery” folder waiting to be stuffed with goodies. I copied over the CSS from the top of the designerwall page, I have the template example (glossy) I would like to work with, but now what? He mentions creating a php file and pasting the CSS and example code into it with some editing. Where do I begin and end with the php? :\ :/ ??

thanks in advance!