How to make dark bits of a image go transparent?

I have a sun image like this that I want to put on a plane and dangle in the sky:

However the textures’ blend setting doesnt seem to be working. In my experience, setting it to ADD or LIGHTEN mode should make it only mix lighter areas in with whatever is behind it. But it only presents the image, complete with black edges. How to make this happen?

Correct, but “behind it” in this case means object’s material. Your best bet is to add proper alpha channel, enable Transparency in material (set Alpha to 0) and then set texture’s influence to color and alpha (leave blend mode at mix).
Or you can use original texture twice, one affecting only color, another affecting alpha, but with ‘RGB to intensity’ turned on.

Hi there, I actually did very close to what you suggested first off actually (but mostly through experimentation). However i DID change it from “mix” to lighten, as i’ve found against the sky this is going, much of the glare around the object is lost otherwise. Thanks for the help though!