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I assume ".jpg" is a real image:

$(function() { // $(function(){}) is a shortcut of $(document).ready(function(){})
 
 var $activeImg; // Maintain a reference to the last activated img
 
 $(".individualImagebox img").click(function(){
 if (!!$activeImg) {
 $activeImg.attr("src", ".jpg"); // Just replace the src with ".jpg"
 }
 $activeImg = $(this).attr("src", "Red.jpg"); // Replace the src with "Red.jpg", and update $activeImg reference
 });
});

I don’t know exactly what you are trying to do but if possible, you should toggling a class instead of modifying the src attribute.

$(function() { // $(function(){}) is a shortcut of $(document).ready(function(){})
 
 var $activeImg; // Maintain a reference to the last activated img
 
 $(".individualImagebox img").click(function(){
 if (!!$activeImg) {
 $activeImg.attr("src", function(i, src){
 return src.replace(/(.+)Red\.jpg$/, "1ドル.jpg");  });
 }
 $activeImg = $(this).attr("src", function(i, src){ // replace attribute and updates active img reference
 return src.replace(/(.+)\.jpg$/, "1ドルRed.jpg");
  });
 });
});

I don’t know exactly what you are trying to do but if possible, you should toggling a class instead of modifying the src attribute.