The Flickr api key can be added in the settings of Slickr and this is also the place where you customize the software. It is possible to display images of a specific user, a group, from everyone with the possibility to customize this request further by selecting Recent pictures, tagged pictures or Interestingness. Last but not least it is possible to use a local folder instead and display the pictures of that folder instead.

This is however not all that can be customized. You can select a minimum width and heights, a maximum size, the time between picture changes, size of the cache and several other settings.

Slickr will automatically add itself as the screensaver in Windows if you select the Screensaver option from the start menu.