Use
<a href="[+maxigallery.path_to_gal+]big_[+maxigallery.picture.filename+]">Download full res version</a>
to refer to the big version of the MaxiGallery image in any of MaxiGallery templates.
For instance, if you are using Lightbox (lightboxv2) the code in file assets/snippets/maxigallery/templates/gallerypicturetpl.html is originally
<a href="[(base_url)][+maxigallery.path_to_gal+][+maxigallery.picture.filename+]" rel="lightbox[maxigallery]" title="[+maxigallery.picture.title:htmlent+] - [+maxigallery.picture.descr:htmlent+]">
In the lightbox window, show link to the big image by adding
<br /><a href="[+maxigallery.path_to_gal+]big_[+maxigallery.picture.filename+]">Download full res version</a>
at [...]
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