You can use the trick of pulling the modal extra field separately. How would you directly output individual extra fields in your K2 overrides? Simple. Just do something like this (e.g. in item.php) to get the extra field name:
$this->item->extraFields->EXTRAFIELD_ALIAS_HERE->name
To get the extra field value you would simply do this:
$this->item->extraFields->EXTRAFIELD_ALIAS_HERE->value
OR
$this->item->extraFields->EXTRAFIELD_ALIAS_HERE->rawValue (for date type only)
Simply replace EXTRAFIELD_ALIAS_HERE with the actual alias of the extra field you wish to output.
If you get the value, it means you get the image path which you can then insert in a new <a> tag and apply your own modal popup.
