You are on the right track on how to achieve an Image Column in the GridView part of the RadGanttView.
To accomplish this we need to add a ColumnDefinition with a custom CellTemplate to the RadGanttView. In the CellTemplate however we should use FormattedValue property instead of the TaskImage property.
Please find attached a sample project demonstrating how to achieve this. I have created an ImageGanttTask that extends the GanttTask by adding an ImagePath property. Then in the xaml I add a ColumnDefinition to the RadGanttView with the custom CellTemplate. This template contains an Image bound to the ImagePath property.
Check out Telerik Analytics, the service which allows developers to discover app usage patterns, analyze user data, log exceptions, solve problems and profile application performance at run time. Watch the videos
and start improving your app based on facts, not hunches.