When I have two images side-by-side ---i.e., subfigures---, how should I make them work togheter? Should I force them to be the same height or the same width? (keeping the aspect ratio, of course).
I would say it depends on the layout of the elements. If they're in rows, keep the height the same. If they're mostly in columns, keep the width the same. If it's a grid, keep the height the same since the text underneath gives a buffer, and the differing widths will be less noticeable.
Then again, if you're able to crop them to the same dimensions, that would be ideal.