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I need an API to change the image of that cell dynamically.

You can use setValue(val) method of corresponding grid cell (get it using yourGrid.cells(rowId,celInd)), where val is list of values separated by ^:

parent_id^Label^children^imageFileName

parent_id - id of parent row

Label - node text

children - 1 if exist, 0 - if not

imageFileName - image file name

I hope this information will be enough for you. But you also can have a look at javascript calendar open source and ajax toolkit progress bar.