The photo field is varchar(255) so it will hold the URL of the photo, not the photo itself.
You should be able to add it to the User Profile editing page without hacking any core code at all.
You’ll want a plugin tied to OnUserFormRender and OnUserFormSave.
Since the photo field is already there, you won’t even have mess with extended fields.
In a plugin tied to OnUserFormRender, this code will add a new field (in this case for a tree_root_id setting):
$fields = '<div class="x-form-item x-tab-item">
<label class="x-form-item-label" style="width:152px;"><b>Tree Root</b></label>
<div class="x-form-element">
<input type="text" name="TreeRoot" value="'. $v .'" class="x-form-text x-form-field" />
</div>
</div>';
$modx->event->output($fields);
I think the hard part will be implementing the photo upload part of the form.