@BlazingCobaltX congratulations! you did it!!
one (1) sincere compliment: even if it's "just a mockup", you fully executed the idea! there's the items, they're on the shelves, they're aligned with the image background! this is a great base for you to work from later when you have more time, and there's lots you can do to dress it up now that you have the basics done!
one (1) sincere criticism: you've made more work for yourself than necessary by doing inline styling on every individual image instead of using css! put that in your stylesheet instead!
instead of
<img src="/images/telecaster.jpg" width=100px height=auto>
<img src="/images/surf.jpg" width=100px height=auto>
<img src="/images/daf.jpg" width=100px height=auto>
etc...
in the html, you can put
#shelf img {width:100px;
height:auto;}
to apply width=100px and height=auto to all imgs in the "shelf" div. then you can just put your img srcs in and not bother with the resizing.
also, it seems like i actually know how to do some of the stuff you want to do here with flex boxes and whatnot. if you'd like, i'd be happy to slap together a commented codepen with some more of the things you want, but i know it's also fun to figure out things on your own! (and, as you said, you don't have time at the moment!) (and it's not like i'm an expert or anything haha)