Whether you are using a trial version of Tableau Desktop or are using Tableau Reader, it is possible to print the dashboard. Go to the File menu in the upper left hand corner and select Print. It can also be printed to PDF if desired.
If you want to export the entire image of a dashboard, go to Dashboard-->Export Image...
If you want to export only one sheet on a dashboard, select the desired sheet, the go to Worksheet-->Export-->desired output.
Hope this helps!