How do I ensure that my Photos images are displayed in the image browser?

To ensure that Posterino can find you Photos media library:

Please make sure that the media library you like to use in Posterino is marked as the "system library":

1. Quit Posterino
2. Start Photos
3. Open Photos > Preferences... > General
4. Click the Use as System Photo Library button:
   Photos preferences.
5. Start Posterino

You should now see the images from your Photos media library in the Posterino image browser.