You can try gparted, which is a bootable partition manager which should be able to extend your recalbox partition.
I would not recommend that option, though, as it can screw things up.
From my experience, I'd highly recommend mount all the roms/art on an external USB of some sort. Mainly because if your installation gets corrupted (which happens more than you think) you better maintain and keep backups, otherwise you're going to have to download all of your roms, scrapes and artwork all over again.
I don't want to throw the developers of recalbox under the bus, as they put incredible amount of their time into developing this wonderful system for free, but they have a history of falling back on their promise of never having to do a fresh build of recalbox between versions. Just keep that in mind when a major release comes out, that there's always a possibility that you'll have to flash another image when the next major release comes out.
If you're insistent that you need the art on your installation drive, you can always create a partition on from the unallocated and point recalbox to the new partition. That's a quick setting change in the recalbox menu.
Hope this helps.
