For small images this is definitely a smart idea. Or, if you can use vectors or SVG images.
This would require getting the <svg.. image data within the inner HTML of a rectangle. This may result in a faster load time since fewer http connections will be required.
If want bitmap images (larger JPEGs for example) referenced remotely, you can insert regular img src code within the inner HTML of an element and reference the image on a remote server. Just make sure that the server supports https since most ad servers require that.