Searching for images is possible, the access is a bit hidden (and the forum internal search works best with single words).
To search images: go to the gallery. Near the top of the page, on the right hand side, part way down is the word "Search" (next to "Mark albums read").
If you click on the word "Search", you get a search dialog, and you can enter words like "spinnaker".
(Do NOT use the "Search...." box in the blue banner; that one is for forum posts only.)
From what I can tell, the search looks for both words in the "image name" and the "comment" section for the given image.
Here's a direct link to the search page for the gallery:
http://forum.daysailer.org/forum/gallery/search.php. As you can see, it allows you to limit the search to a particular user's images.
Here's the search results that are pulled up when you search for "spinnaker":
http://forum.daysailer.org/forum/gallery/search.php?keywords=spinnaker&terms=all&username=&sk=t&sd=d&e=0&submit=SearchIf people leave the image name the same as their local file name (for example "DSC 0138 640x480") and do not enter a description, then the search has nothing to go by; however, I easily found about 20 images related to spinnakers. If you have anything in your galleries that you think is useful for others, make sure to use a keyword as part of the image name and add a short description in the comment. (Looks like you can edit name and description for existing images as well - haven't actually tested whether it saves them to the same index number, but would expect so).
Now, Tim is correct, it's not possible to search for "DSII rigging" and expect to get reasonable results. However, "Rigging" by itself gets about 2 dozen hits, some more useful than others.