There isn't an event that fires when an attribute is changed on the HTML elements, so the tooltip manager cannot know of this. It goes through the page when the page loads and this is when it takes the values. What you can try is to call its tooltipify() method after you are done changing the attributes:
"new alt attr"
where img1 is your last image and rttm1 is the RadToolTIpManager. You only need to make sure this is executed after the Sys.Application.Load event
so that you can get a reference to the RadToolTipManager (it is a script control and this is the point when the object is available).
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