Using the picture attribute with media queries lets you do very cool things. In the initial view of this page, for instance, smaller project pictures are downloaded. But if you're viewing the page from a larger viewport that can support a larger picture, the larger picture will be downloaded too so you can see it in the modal window!
Milky Way galaxy
This is a second example window. Look at now clearly you can see the Milky Way!
Arches National Park
Check out this view from Arches National Park! Pretty cool, right? But how awkward would it be if this guy in the photo were by himself and his flashlight battery died? He might have to wait until morning to find his way out...