You will need to use CloudExport to make it a cloud object, visible in the file explorer and shareable or downloadable:

CloudExport[Image3D[RandomReal[1, {5, 10, 10}]], "JPG", "SPRITE.jpg"]

You can use Export and other file system commands in the cloud, but they remain on a file system available only to you in Wolfram Language while running code in the cloud, visible via FileNames etc. To promote them for more general use, you need to create cloud objects via functions like CloudExport, CloudPut, and CloudDeploy.

unfortunately I’m still not seeing the file in the right-hand Cloud Files pane. :-(

I can think of a couple of explanations for that, but the simplest would be if you don't mind sharing the exact instruction you used for that, you can obfuscate any data or names you like, but I'm interested in how the CloudExport function was called.