Great news, everyone! I've figured out how to make any picture annotatable!

Annotations are triggered by an image's size. I peeked at the dimensions of Seoul Sister's images, which were 970 pixels wide and about 550 pixels high (I think it's the width that's really the key here).

Basically, this means you can trick Kinja into making even a small image open to annotations. All you have to do is use a basic image editing program (even MS Paint works) to create a canvas whose dimensions are large enough for Kinja to recognize. Then, paste the picture you'd like to annotate onto that canvas, and save the whole shebang as a new image. Boom! Now even your tiny pic can be annotated out the wazoo.

For this image, I created a canvas in MS Paint that was 970 x 660, and pasted a picture of our fine, swirly-headed friend here onto it. Now we can annotate him to our black little hearts' content!

PS.
I did ask a tech what the specific dimensions for an annotatable image are, but I haven't heard back yet.