According to Detailed Plugin Guidelines:
7. Plugins may not track users without their consent.
Plugins may not track users without their consent. In the interest of protecting
user privacy, plugins may not contact external servers without
explicit and authorized consent. This is commonly done via an ‘opt in’
method, requiring registration with a service or a checkbox within the
plugin settings. Documentation on how any user data is collected, and
used, should be included in the plugin’s readme, preferably with a
clearly stated privacy policy.
So it all depends on what that image was exactly and was it used. Gravatars are external. FontAwesome files as well…
So you can include external resources. But I don’t think you should, unless it’s necessary. (And tracking users is not a good reason).