First, make sure you've created the audience you want to use as an image pixel.
Once the audience is created, head 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):
Note: Exporting your audiences as an image pixel is an Enterprise only feature. If you are interested in joining our Enterprise program, please contact firstname.lastname@example.org.