i've stopped using tables for image galleries altogether. instead, i prefer to just spit out a bunch of divs and float them with CSS. eg, see this gallery. the advantages are that it is simple to generate (you can just use a plain <tmpl_loop>) and if you resize the browser window, they will reflow to fit. this allows the user to take full advantage of a big screen without making users with small screens scroll horizontally. if you still want to limit it to a certain number of columns, you just enclose the whole thing in a div with a fixed width. if you have images with irregular dimensions, you need to force the enclosing divs out to the maximum width and heights to make it look neat.
i used to try to do these with tables and it involved all kinds of annoying code to add in empty entries for the remaining table cells.