I haven't tried but wouldn't this help?:
http://api.jquery.com/nth-child-selector/
I mean the number of <td> and even <tr> is known so it should be possible to get the nth tr with the nth td within to change the value.
the problem is the tables containing the items I want to select are also not uniquely identified, so if I use nth:child I may end up selecting items in other backend tabs or panels.
where n is the row number. It's complicated by the fact that some of the rows of the containing table for the data for each image contain further tables, but the method above does appear to work.