That depends. If the overlapping image is simply an image and is either absolutely or relatively positioned (sometimes, in some browsers, even if it is not), what you need to do is give your pop up a higher z-index in its style, ex:
HTML Code:
<img src="whatever.jpg" style="z-index:1000;">
Depending upon the stacking order of the page, even a z-index of 1 might be enough. Go for the lowest value that works.
If the overlapping image is in an iframe or is flash, other methods must be used and success is not 100% guaranteed.
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