How can I create an ASP.net site that shows users online with their image by distance?

 Hi everyone,
 I'm looking to design a website that displays my members online and also by distance such as  10 , 25, 50, 75 and 100 miles away from whatever my user inputs as their zip code?
 
I found a script that will show users online, but what would be a good SQL query to show users online by zip code? Surely there is an easy and efficient way to do this.

Could someone guide me in the right direction as to how this is possible?
 

 
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