I have a grid of a variable number of elements. Say 5 images per row and a variable number of images. I need to determine which column (for lack of a better word) each image is in... i.e. 1, 2, 3, 4 or 5.

In this grid, images 1, 6, 12 and 17 would be in column 1 while 4, 9 and 15 would be in column 4.

1  2  3  4  5 
6  7  8  9  10
12 13 14 15 16
17 18 19

What I am trying to do is apply a background image to each element based on it's column position.

An example of this hard coded and inflexible (and if I'm barking up the wrong tree here by all means tell me how you'd ideally accomplish this as it always bugs me when I see someone ask "How do I build a gold plated, solar powered jet pack to get to the top of this building?" when they really should be asking "Where's the elevator?"):

switch (imgnum){
                case "1" : case "6" : case "11" :
                    value = "1";
                    break;
                case "2" : case "7" : case "12" :
                    value = "2";
                    break;
                case "3" : case "8" : case "13" :
                    value = "3";
                    break;
                case "4" : case "9" : case "14" :
                    value = "4";
                    break;
                case "5" : case "10" : case "15" :
                    value = "5";
                    break;
                default :
                    value = "";
            }

            $('.someclass > ul').css('background','url("/img/'+value+'.png") no-repeat');