Different aspect ratios on individual pages

Site URL: http://www.stevekephart.com

I'm extremely new to this and am trying to figure out how to build my art website and storefront.  Some of my drawings have different aspect ratios, 4:3 and 3:4 but the web builder only lets me select one and defaults that to all the pages.  This means only part of the drawing is displayed on some pages. 

I searched for this issue and saw responses about inserting a code on the pages that are affected.  What code would I need to insert for this and how would I do this?  I believe they specified "page header", but all I see an option for is "site header".  Like I said, I'm very new to this.  

Thanks for your help!

To open Page Header, hover Page Name on Pages > Main Navigation or Not Linked > Click Gear icon > Advanced > Look right

Thanks for the reply.

Wouldn't any injection be applied to everything within that section?  For instance, in my case I have the page called "Shop".  Then within that I have two separate sections called "Original drawings" and Art Prints".  And then within those I have individual items.  I'm trying to have different aspect ratios on each of those individual items.  

Also, what code would I use for 4:3 ratio?   And what about a 3:4 ratio?

Please see the following. I think it is close to the effect you want.

Let us know how it goes.