To uninstall Photo Desktop first make sure it is not running, see other Faq on how to quit Photo Desktop.
You can then simply move the application to the Trash, you can probably find it in your Applications folder.
You may want to perform some additional cleanup,
remove the Photo Desktop folder found in the Application Support folder of your Library,
it contains some additional cached images.
Additionally you remove the AtCoreToolbox.framework, which you can find in '/Library/Frameworks'.
You can use the help documentation bundled with the Editor for a more detailed overview.