There are graphic software which provide you all the tools to create a cover, but you need to know how to use them and what is needed to create a technically correct cover. Maybe there are even apps which create covers for different platforms. There is also adobe photoshop available.
For transfer you can just use ftp clients like Total Commander, to copy cover from smartphone to xbox. Or you can just upload them via browser on unity.
İf you dont have a network connection at all i recommend at least to built up one with a router, which you will need to get.
Also on some android devices, you can get OTG cable to connect a USB flash stick to your phone and transfer the files on it with the file browser, a good one is ES File Manager.
Also another idea is to use the mobile hotspot, if your xbox has wifi, so you connect it with that, and use the ftp clients for that.