If you replace the {size} portion of the string with a number it should generate the avatar.
You could perhaps create a computed property, that takes the avatar_template and replaces it with a number like:
@discourseComputed("user.avatar_template")
avatarSrc(avatarTemplate) {
return avatarTemplate.replace("{size}", "75");
},
and then call that in your template:
<img src={{avatarSrc}} alt="the profile photo of the user"/>