If you wish to use an image pixel to add users to your audiences, you can easily do this by exporting an existing audience to an image pixel.
First, make sure you've created the audience you want to use as an image pixel.
Once the audience is created, go to Manage > Audiences.
Find the audience you want to export and check the box next to it:
Then click Actions > Export Pixel.
The pixel code will be displayed for you to copy and use as needed (your code will look different):