this problem comes from using a sub domain.
when you are at the subdomain:
m.senecagun.com, the files at the root of the site are not visible to the subdomain.
the subdomain cannot access files at senecagun.com using a relative path:
../dbimages
in a sub domain, you would need to access the files using a system path. I changed the code to:
<!-- Script for checking that the image is correct--><?php if(file_exists("/homepages/8/d429129137/htdocs/".str_replace(".JPG",".jpg",$row_WADAinventory["image"]))) { ?><img class="WADAResultThumb" border="0" src="http://senecagun.com/<?php echo $row_WADAinventory["image"]; ?>" alt="<?php echo $row_WADAinventory["model"]; ?>" /><?php } else { ?><img class="WADAResultThumb" src="../images/default.jpg" border="0" alt="<?php echo $row_WADAinventory["model"]; ?>" /><?php } ?></a>