I decided "mirror" quite quickly when I first saw it. The photons "dieing" refers to their being absorbed by something they hit, and the mirror largely prevents that and sends them back out into safe space.
It may have been more obvious if it had been a large round/oval shape instead, which I think is much more representative of a mirror. The test we used for deciding representative members of a category in cognitive science was, If you tell a small child to draw it, what will they draw? I think if you told a kid to draw a mirror, it would be oval instead of rectangular.
I haven't seen an actual oval mirror in years and years, and I see lots of rectangular ones -- but I'm well aware that the canonical form of something in a society doesn't always match what's most common. And my personal sample is not representative, and my attention may be uneven.
Also, while I have seen oval framed art, it's vanishingly rare, which might help with discrimination.
Given the decorative corners on the frame, I perceived it to be an empty picture frame.
Ah. And. In the walking frame, there is one for... *gets confused for a moment thinking he saw something proving frame* Okay. In the walking frame, the object in question has speckles on the inside, so is not likely to be an empty picture frame as there are no speckles in the same general area around it. There are also two body lines, and the rear arm, which is holding the front of the object in question, is also doubled, while the front arm, which is holding the rear, and wrapping its hand around to the front further forward of the body, is missing.
Mirror it is. :)
Which completely changes the comic for me. Thanks for asking!