When you're creating a campaign, in the Creatives step, you will see, when uploading a picture, an option to define which placements will use which picture. 

This is Asset Customization: using specific pictures (with different ratios for instance) for each placement, to make sure you're making the best out of your assets! 

If you've selected a placement that is not compatible with this feature, we'll warn you in advance! 

But if you're missing one placement that is compatible with Asset Customization, we'll warn you as well: 

Once you're done, let's get to the feature itself!

In the Creative Step, after uploading your picture, here's where you can leverage this option: 

Opening the drop-down will let you choose the placement for which you want to use a different image:

Once you've chosen your placement, click on the option on the right to choose the image you want to use:

You can then select an image the same way you would add one to your creative.
Any time you add one, you will see it added to the list, like this: