You can subscribe to the CurrentImageChanged event of the image history to be notified about changes in the current image.
.ImageEditorUI.ImageEditor.History.CurrentImageChanged += History_CurrentImageChanged;
Please note the event is raised when the tool you are working with is committed, i.e. when the image is actually changed. You can also take a look at this SDK example
to see how to work with the ToolCommitting and ToolCommitted events.
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