Use the gsutil cp command to create a folder and copy the image into it:
gsutil cp gs://my-awesome-bucket/kitten.png gs://my-awesome-bucket/just-a-folder/kitten3.png
If successful, the command returns:
Copying gs://my-awesome-bucket/kitten.png [Content-Type=image/png]...
Copying ...my-awesome-bucket/just-a-folder/kitten3.png: 164.3 KiB/164.3 KiB
You've just copied your object into a new folder in your bucket.