ASP.NET set image control by javascript

Hello experts.

I have a popup page which is passing back a value and im trying to set my asp image control with this value.

First of all, the user clicks on the image control which brings up a popup page with images, they then click on an image and it passes the value of that image into the image control.

I then want to use those values in the asp.net code behind. So the c# can take the passed back value from the image control.

The problem im having is the values are not persisting when the button is clicked. I passed the value into the image title, so then in the code behind i have tried to get the value tooltip but it doesnt exist by then.

How can I do this?

Thanks