How to add avatar to your emails
E-mail is great, but making your email stand out among the other attention-seeking items in your recipients inbox can be hard. One way to increase recognizability is to add a profile picture or avatar to your email.
Unfortunately, there is no standard for adding sender images to emails - or for that matter any non-standard way, that I am aware of. Thus email clients have had to come up with their own solutions.
In order to have your avatar show up in Gmails interface, you have to sign up for a Google (Plus) account using the relevant email address.
After doing so you can access and change your profile image in the account menu found in the topright corner of pretty much all Google properties. It even has a link to change the image right in the dropdown, just click your current profile image.
Other email clients, like Polymail or Airmail (and many online services) often use Gravatar to look up avatars for email senders. Gravatar is a free service, owned by Wordpress, providing "Globally Recognizable Avatars".
If you sign up there, you'll be able to manage avatars for a number of different email addresses that you add to your account.
I haven't been able to find a consistent way to add profile images to recipients using Outlook, short of something to do with ActiveDirectory and then only usable inside a specific organization. Please get in touch if you know something, I don't.
One final notice
PS: Do note, though, that your recipients may be able to override your avatars if they have added a custom image to your contact listing.