I think I've solved this by changing the template so that it sets the height and width of images to fairly large percentages of the page regardless of their original size. Right now, the template only scales up, not down, via these lines:

\def\maxwidth{\ifdim\Gin@nat@width>\linewidth\linewidth\else\Gin@nat@width\fi}
\def\maxheight{\ifdim\Gin@nat@height>\textheight\textheight\else\Gin@nat@height\fi}

I've commented these out and put in this line just below them, in place of the corresponding original:

\setkeys{Gin}{width=\linewidth,height=0.9\textheight,keepaspectratio}

I also use the Boadilla theme which gives smaller margins, it looks like, at least on the left and right.

This seems to work OK with columns, too, though I haven't played around too much with it. In-line images require height and width attributes in order to work (though that may be true with the original template as well).