Compositor doesn't affect Alpha plane In this render, the butterfly is an Image imported as plane with Alpha.
The non-transparent parts of the image have an Emission material with 18 Strength.

The Sphere above the butterfly has Emission material with 18 Strength.

However, once composited, the Glare effect from the Compositor seems to only work on the Sphere and not on the Alpha PNG.

How can I change my Node setup so that the Image-Plane also be taken into consideration by the Compositor?
I think the short answer is that the image plane is indeed being taken into consideration.  I checked that by altering the colors of your image so that the butterfly is white instead of pink.  In this case, the glare node does affect the butterfly.  

I think your best bet is to really crank up the strength of the emission shader for your butterfly (I tried 200), which will give you the glare. If you want more control over the color of the butterfly image itself, you could experiment with render layers--use one layer for the butterfly image to be "glared", another layer for the color to be composited over top.