You have all kinds of problems on this page, but there is at least one blocker:
You are calling your PopUp method like this:
PopUp('enlarged','websiteimages/225trans.gif', 'caption',' ');
However, your "PopUp" implementation actually takes 6 parameters, not 4. You are trying to use the last two input parameters (xos,yos) in a calculation, but they are not defined. As a result, the calculations are computed as "NaN" (not a number). Then, right here:
You are trying to set some values with garbage data, and IE is throwing an "Illegal Argument" error. You need to get this fixed first before you work on anything else.