To answer your questions:
what is the data type for the new column you added? MediumText
in the update page, did you add a form element for adding the alt text?
<li class="formItem">
<div class="formGroup">
<div class="lineGroup">
<div class="fullColumnGroup"><label for="prodImageAltText" class="sublabel" >Product Image Alt Text:</label>
<div class="errorGroup">
<div class="fieldPair">
<div class="fieldGroup">
<span>
<input name="prodImageAltText" type="text" class="formTextfield_Medium" id="prodImageAltText" tabindex="11" title="Please enter a value." value="<?php echo((isset($_GET["invalid"])?ValidatedField("productsupdate","prodImageAltText"):"".$row_WADAproducts["prodImageAltText"]."")); ?>" size="100">
</span>
</div>
</div>
</div>
</div>
</div>
</div>
</li>
Did you update the update record behavior so it is storing the alt text? How?