Can you please let us know how you add assets to GWD? Did you add the images to the assets folder on disk or actually importing them to GWD?
Adding them to the assets folder on disk won't automatically add them to GWD UI. We have a feature request to reload the Library panel for that use case but we haven't had a chance to implement it.
If you import them using GWD UI or drag them to GWD, they should show up properly in the UI. If they don't show, it sounds like a bug and we need to further look into how to reproduce it. For example, where do you save your file? Is it on a network shared folder or locally?
Please let us know.
Google Web Designer team