There is alt text unless I accidentally did something

By the way, is there any reason, why IPTC fields are copied to thumbnail, but not to preview image?

alt is on the individual image page, but not on the page displaying contents of a category.

eg
http://cascoly-images.com/pix/image-type/elephants-images/

uses the following for the image:

  <a class="search_result_preview" title="Hunting tiger from elephant" href="http://cascoly-images.com/pix/image/hunting-tiger-from-elephant/">
              <img class="search_minipic" src="http://cascoly-images.com/pix/wp-content/uploads/symbiostock_rf_content/434-minipic.jpg" />
            [/url]

so the title is being used but not the alt

I just did a quick search and th consensus, backed by some info from google is that crawlers use the alt more than the title;  there's some concern that using both may count as keyword stuffing, but others say it's ok as long as the text is relevant 

if I had to use only 1, i'd go with alt

I think it's important to have the alt on these category pages since they are more likely to be crawled & indexed than indiv image pages

Actually I specifically left off alt text on preview images to keep the main image as most relevant, and not water down its unique-ness on the site. I was thinking of putting something more generic on the pointer pic like "preview for image number 1234".

that's fine on the preview page -- I was referring to the pages that result from clicking on a category or doing a search where you have multiple images, none being primary

Just fixed it. It has "stock image #----"