[smalltalk]

Usually, an image is opaque. A rectangle.

What to do if you want a circle, for example against a white background?
-> Draw a green circle and make the outerior white -> e voila.

If you now change the white background to, maybe red, what will you see?
-> A green circle in a white rectangle against a red background.

Nowadays, images can be transparent.
Again, draw a green circle but now make the outerior transparent
-> this gives us a green circle against -any- background.
Please notice the sharp border of the circle.

The current logo also has a drop shadow, - no sharp borders- ,which blends from almost black to grey to white/transparent. You have to decide at design time, which background color you want to have or you'll get ugly artefacts.

Just put the logo on a blank page and change background color of the page. You'll see what I mean.