Yes, it is, though you will need admin rights to the Tableau install directory, and you shall need to edit a file.
So you want to edit the Sample Workbooks section of the Start Screen?
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Adding your own sample workbooks can be a great time-saver, far faster than saved data sources:
For works requiring Superstore and/or Adventureworks such as forum, tutorial, blog or teaching works, I have a workbook that has a pre-configured data set, just like a saved data source. But unlike a saved data source, this workbook already has my logos, parameters, a blend, some actions, a couple of starter charts, and a dashboard template, all in place and ready to go. By setting the workbook as read-only, not only do I have this workbook at my fingertips from every instance of Tableau I open, but being read-only, the OS will force a Save As, preventing me from accidentally overwriting something else.
Firstly, you need a workbook that you plan to use, and a 480px x 360px png image of one of the dashboards to use as the thumbnail, once you have these, navigate to your Tableau install directory: (Windows default) C:\Program Files\Tableau
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