Use Tumblr's built-in lightbox to display high resolution photos. I don't necessarily recommend using this anymore as it unnecessarily relies on jQuery, which you may not need. For a vanilla JS version, see: https://gist.github.com/edadams/5ce1ec3d0b1f69e80724af7eb3b606f4

tumblr-lightbox-for-photos.js

Hey! Just to let you know there is a much better version of this script that doesn't require jQuery here:
https://gist.github.com/edadams/5ce1ec3d0b1f69e80724af7eb3b606f4

For posterity, and because this Gist got unexpectedly popular for a random code snippet, I will preserve the original below. Please do know that I've figured out a much better way to do this, which is above.

$(function(){ // shorthand document.ready()
    $('.make_lightbox').each(function(){ // this is just an element I let them click, it carries a series of data- attributes.
        $(this).on('click',function(){  // when clicked. this is the newer jQuery click() handler that's only in v1.8+ so that may be something to note.
            var lbArray = []; // create blank array.
            var arrayContents = {"width":$(this).data('width'), "height":$(this).data('height'), "low_res":$(this).data("lowres"), "high_res":$(this).data('highres')}; // make set of the data- attributes. 
            lbArray.push(arrayContents); // push the set into the array.
            Tumblr.Lightbox.init(lbArray,1); // trigger Tumblr lightbox. the '1' is the number of photos it expects. specific to how tumblr has things set up.
            $('body').toggleClass('tumblr_lightbox_active'); // use this to prevent the user from scrolling.
        });
    });
});


// Example usage:
{block:Photo}
    {block:HighRes}
        <div class="make_lightbox" data-width="{PhotoWidth-HighRes}" data-height="{PhotoHeight-HighRes}" data-lowres="{PhotoURL-500}" data-highres="{PhotoURL-HighRes}">View high resolution version</div>
    {/block:HighRes}
{/block:Photo}


// and a bit of css to stop them from scrolling.
// according to a comment, this is optional
body.tumblr_lightbox_active {
    overflow: hidden;
}

Thanks so much for the response! I'm looking into other lightboxes now, but this was a good learning experience.