Try to add the field as hidden input field and use javascript to change its content, eg. something like:
<input type="hidden" name="[+maxigallery.fieldnames.category+]" value="[+maxigallery.picture.category:htmlent+]" />
<select onChange="document.editform.[+maxigallery.fieldnames.modified+].value='yes';document.editform.[+maxigallery.fieldnames.category+].value=this.value;">
<option value="[+maxigallery.picture.category:htmlent+]">[+maxigallery.picture.category:htmlent+]</option>
<option value="event">Event</option>
<option value="product">Product</option>
<option value="promotional">Promotional</option>
</select>
"He can have a lollipop any time he wants to. That's what it means to be a programmer."
that did it! thanks so much for the help and for making MaxiGallery in the first place