Image previews only appear in File Chooser after opening a folder containing the image file with Nautilus. Viewing Recent files with Nautilus also doesn’t load their previews
No, there is no workaround. GTK does not generate thumbnails by itself, as the code that spawns the thumbnailers is currently part of libgnome-desktop.
There are still unresolved questions when it comes to API, notification of thumbnail creation and failure, and where the dependencies lie, especially now that (for security purposes) the thumbnailing is done within a sandbox: Unix Thumbnailing API (#4379) · Issues · GNOME / gtk · GitLab