inconsistent HTML/PHP coding within different pages

It seems I have few post that suddenly changed its default coding, which causes my header image to be replaced with random image within the post. The default code should be

<header id="branding" role="banner">
<hgroup></hgroup>
<a href="https://websiteurl/"><strong><img width="1000" height="288" alt="" src="websiteurl/wp-content/images/title_01a1.jpg"></strong></img>

But instead, couple of pages have

<header id="branding" role="banner">
<hgroup></hgroup>
</a><a href="websiteurl/"><strong><img class="attachment-post-thumbnail wp-post-image" width="1000" height="288" alt="Pteraiden-Oh01" src="https://websiteurl/wp-content/images/Pteraiden-Oh01-1000x288.jpg"></strong></img>

Is there anyway I can edit these codes? I can’t seem to find a way and I couldn’t find any plug-ins that can do this