That particular template is a fixed-width template, and as such, would require a fair amount of rework to update it the way you're looking to do it.

This can indeed be done, though, if you head to Admin > Layout > Advanced Layout Options. There, you can completely modify the HTML and graphics.

Without doing any HTML modification, your only alternative is to change your site to a different Drop-In Template.

That said, for the boss kills, you could upload them to the gallery, then use the BBCode links to redirect them on click to the gallery, where users would then be able to see the full-sized picture.