Yes. File URL must be absolute or relative.
Absolute path example: http://YourSiteURL/path/to/your/file <<< This is always constant in all pages.
Relative path example: ../../path/to/your/file <<< This is changed by page URL.
Below codes are absolute example.
1. Case of stand alone HTML (Hard coding): This is most easy.
<div class="cover-pic">
<img src="http://YourSiteURL/modal.png">
</div>
2. Case of stand alone HTML (PHP): This is difficult. (Require qa-include/qa-base.php)
<div class="cover-pic">
<img src="<?php echo qa_path_to_root();?>modal.png">
</div>
3. Case of inner theme.php of your theme: This is relatively easy. I recommend.
$html = '<div class="cover-pic">';
$html .= '<img src="' . qa_path_to_root() . 'modal.png">';
$html .= '</div>';
$this->output($html);
Code example of Q2A logo image is qa-include/qa-page.php L652.
There are other correspondence methods.