Store page

I have product categories as page. And each category filled with products.
I did attach the given code next to product and product id. It works fine.
My question is, if I have category pages filled with products why do I need to create store and through the store I have to upload pictures again?

My product pictures reasonably large. If I use store will it give the same size picture?

Also other issues like checkout and thank you pages.. Is there more detail instruction for it?
my website:http://lietissimo.com if you check you can get better idea about the pictures sizes and functionality.

Thank you.

https://ww.wp.xz.cn/plugins/simple-e-commerce-shopping-cart/