First, install the add-on on the Google Workspace Marketplace. Once installed, select an image you want to filter. Then start Image Filters by choosing 'Start' under 'Image Filters' located under the 'Add-Ons' menu on the top menu bar. A sidebar will open and after a few seconds of processing, you can choose the filter you want to apply. After you've chosen a filter, it will typically take 10 seconds for the image on the slide to be updated.
After you've applied the filter to an image, click the filter preview again. A bar will appear that follows your mouse as you move over the preview, allowing you to adjust the strength of the filter. Right-click to apply, and left-click to cancel.
This is typically a temporary problem with the Google Slides infrastructure or your connection. Try again by closing Image Filters and reopening it from the Add-On menu.
Image Filters is currently free of charge! We do ask, however, that you give us feedback on what you like/don't like about it. Please drop us a line to say hi!