I think this is what you are looking for.
You can change the tpl file in assets/maxigallery/templates/gallerypicturetpl.html (that is what mine is named, I may have changed that a while ago but I think it’s the original)
In the first lines I have:
[+maxigallery.embedtype:is=`smoothgallery`:then=`
<div class="imageElement">
<h3>[+maxigallery.picture.title:htmlent+]</h3>
<p>[+maxigallery.picture.descr:htmlent+]</p>
<a href="[+maxigallery.picture_link_url+]" title="[+maxigallery.strings.click_to_zoom+]" class="open"></a>
<img src="[(base_url)][+maxigallery.path_to_gal+][+maxigallery.picture.filename+]" class="full" alt="[+maxigallery.strings.go_to_gallery+]"/>
<img src="[(base_url)][+maxigallery.path_to_gal+]tn_[+maxigallery.picture.filename+]" class="thumbnail" alt="[+maxigallery.strings.go_to_gallery+]"/>
</div>
`+]
I use smoothgallery so if you are using a different one find where in that code it is, but in my case I would change:
<img src="[(base_url)][+maxigallery.path_to_gal+]tn_[+maxigallery.picture.filename+]" class="thumbnail" alt="[+maxigallery.strings.go_to_gallery+]"/>
to:
<img src="[(base_url)][+maxigallery.path_to_gal+][+maxigallery.picture.filename+]" class="thumbnail" alt="[+maxigallery.strings.go_to_gallery+]"/>
I took out the "tn_" prefix to have it call the regular file, not the thumb.
Does that help?