On a previous project, I generated a few custom svg images and uploaded them using the image management tool.
I don't have access to the original files any more, but I do have access to a gateway backup from that project. I have restored the gateway backup onto a temporary VM - how can I extract the images from there, and get them onto another gateway? This post suggested that I can just select Export Project Resources and check the box for images, but I can't see any such box in the export dialog.
They aren't project resources. If you dig around in the config.idb from the backup using an external SQLite tool, you should find them stored as blobs.
resources are also in a subfolder {Ignition folder}/data/projects/.resources but file names are not great so you will need to open every file there to find the one you want