Need a new "Front Page" on the site

I FINALLY got my site moved and working. But I KNOW I need a new theme and a new, refreshing, updated look.
My idea is to make a "front page" that is a collage of what you can do and where you can go on our site. This "front page" has to be easily usable with laptops, tablets and phones. We have 25 level 2 Categories, most of which I would like to be featured in that collage. I have created a mockup of the "look" I want and will display a screenshot of it. This software has a great example of the collage look. It's at https://monosnap.com/en/page/download/windows . It actually has 25 boxes in 4 different sizes.
The pics in the 'front page' collage will have links to the main category page for that image. From there, the user navigates by clicking on the already existing links to level 3 sub-categories, etc.
The background image must be easily changeable so we can feature a new background 'look' on a regular basis. There must be decent documentation in the code so I have a clue what it's doing if I need to change things like the picture, the address to the link, etc.
Above the collage must be the 'banner' that is at the top of the existing Home page (Skydiving Museum and Hall of Fame, with the logo, and the words SKYDIVING ENCYCLOPEDIA.) See http://skydiving-encyclopedia.com/sample-page-2/ to view our existing home page. The only thing missing from this is the slider. It 'would' be nice if the center pic of the collage was a slider but that is not absolutely necessary. IF we can have a slider there, I like meta slider. It works well, and is easy to use and set up. IF you have a suggestion for another slider, I am DEFINITELY open to hearing about it.

PLEASE let me know what I can use to make my HOME page a collage with clickable pics that I can link to my level 2 Wiki Category pages.

The screenshot will follow in the next box.

Thank you.

Mary