Hi,
A custom product field of the type AJAX image is already supposed to display the images direclty on the product page.
But yes, there is no mechanism to display it with a popup system and it just downloads it. That would require some development to have it support fancybox.
I think the modification shouldn't be too complex though.
In the file show_block_custom_main ( which you can edit via the Display>Views menu), you could add the code:
<?php if($oneExtraField->field_type='ajaximage') { ?>
<script>Fancybox.bind('#hikashop_product_custom_value_<?php echo $oneExtraField->field_id;?> a', {});</script>
<?php } ?>
after the line:
<?php echo $this->fieldsClass->show($oneExtraField,$value); ?>
and it should work (not tested though)