After deinstalling UCC from the App Center, the downloaded images are still there in /var/lib/univention-client-boot/.
That's a limitation in the integration of the image download in the UCC app. This has been documented in the UCC manual. The best way to resolve this in the App Center is to drop the download in the postinst and create a UMC module for image management -> Bug 30379
*** This bug has been marked as a duplicate of bug 30852 ***