The "Achievement" (my bad, it acted like a quest) introducing the concept informs you that you can either drag an image into the game, or go into settings to get a file dialog. The achievement only clears if you drag an image, so if you use the file dialog in the settings menu, that doesn't count for the achievement (at least, so it seems, I noticed achievements only trigger if the game is unpaused, and sometimes menus pause the game)