onTextCahnaged event of textbox

dear all;
              i have a button when the user clicks on it a small page(child page) will be opned  the user selects a value then this value is getting pasted on textbox in the parent page.. according to the value on textbox i want to fire onTextChanged event to change an image in parent page dynamically...
  i have done that but  onTextChanged event does not fire at all....  ????????
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