Hi,
To replace an image in your ad with one in the asset library, right click the image you want to replace, and select "Swap image..." from the pop-up menu.
In the dialog, either select an image from the asset library or click "Browse" to select an image from your computer.
https://support.google.com/webdesigner/answer/6184292?hl=en
Thanks
Sukhbir
(Google Web Designer Team)