The image on XenForo's alert system is normally limited to user avatars, not arbitrary images (unfortunately). You can see what I mean if you look at the alert_macros
template. So to get something different than an avatar of a user there, you would need to make some more substantial changes to how alerts are presented. Not impossible, but in it's current form, you specify a user and their avatar is presented and that's your only option for displaying an image there.