heres another idea for this.

edit:
includes/languages/english.php

find the following:
define('TEXT_CLICK_TO_ENLARGE', 'larger image');

you can add another line like this:
define('TEXT_CLICK_TO_ALTERNATE', 'alt image link text');

if you want no text on the link, use:
define('TEXT_CLICK_TO_ALTERNATE', '');

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then edit:
includes/modules/YOUR_TEMPLATE/additional_images.php

replace all the instances of TEXT_CLICK_TO_ENLARGE
with TEXT_CLICK_TO_ALTERNATE