Need to insert an image on the footer of php file but doesn't work out

Need to insert an image on the footer of php file but doesn't work out

I'm not very familiar with php, so I really need help. The following is
the php code. I need to insert an image in the center of the footer, but I
got two problems:
I don't know how to insert it.
The image is stored in server, I don't know the url of it, I'm not sure
which url I could use to locate this image.
Please help!
<div id="g-footer" class="ui-helper-clearfix">
<?= $theme->footer() ?>
<? if ($footer_text = module::get_var("gallery", "footer_text")): ?>
<?= $footer_text ?>
<? endif ?>
<? if (module::get_var("gallery", "show_credits")): ?>
<ul id="g-credits" class="g-inline">
<?= $theme->credits() ?>
</ul>
<? endif ?>
</div>