To use Google Slides in offline mode you will need to make sure that you have Offline Editing for Docs enabled as shown here. You can enable it via the Settings Menu in the upper right corner of the browser window.
Note: We have included the link to the offline mode set up instructions below for your convenience.
Once that is done or if you had previously set up Google Docs for offline mode, then the only other requirements are that you need to have the Google Slides App installed (link below), and be using Chrome or ChromeOS.
Special Note: Keep in mind that the images shown in the top image above were already inserted into the Slides Presentation BEFORE we started editing offline. At this time you will not be able to insert images into a Slides Presentation when in offline mode, so add them in early if you can.
Create, edit and present with Google Slides offline [Google Drive Blog]
Google Slides Web App [Chrome Web Store]
Set Up Offline Access For Google Docs [Google Support Page]
Bonus! Download the Wallpapers Shown Above
Spice And Wolf Wallpapers [Just Akemi…]