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No options available to add different stamps other than the default ones. Also the arrows for stamps should be disabled as they are misleading and of no use
It's not currently possible to install new stamps within Tux Paint. That said:
(1) A large stamp collection is available as a separate download/install
(2) we are considering adding Get Hot New Stuff (GHNS) support, or something similar, to allow downloading of new stamps directly within Tux Paint (apparently not tracked in SF.net project?)
(3) As part of a planned copy/cut/paste feature, the ability to create a new stamp using the selection tool is planned (see #1598377).
But yes, the purple buttons should not be purple when there's only one set of stamps (e.g., the Tux penguin ones that ship with Tux Paint).