I'm not really the best visual brainstormer-- since I can't sketch that well. So basically my brainstroming was just writing out what I think I wanted to do. And then questioning.

I know that I wanted to definitely have an image of earth from outer space, because this is the only possible view that the aliens might be familiar with.

 

So I found this image online

At first I was going to make it very simple with just the earth cut out on the "cosmic latte" color.

 

And I was toying with the idea of adding words, just for visual interest. But then I decided that I already wrote down that I didn't want to include any written language.

So this is what I ended up producing.

It is suppose to represent our galaxy on the right with the image of the earth. --All on the color "cosmic latte" which is suppose to be the average color of the galaxy.

And then since I assumed astronauts would be delivering this postcard to the aliens, ...
The left side is what I would deem "the unknown". The repeating circles are about all the possible planets and objects beyond what we know. I included three rows of because odd numbers are better than even.

 

And then I included the little "doorway" from our galaxy to this unknown area... because we will have somewhat crossed this gap in order to deliver the card to the aliens.