They already cache the jpeg files. At least some of them. Most of my jpegs are cached in memory on my boxee boxes. If I unplug the box, connect it an hour later with no internet connection, the vast majority of the poster art that I created in ember still shows up. For the titles with no poster art there is just a gray box with the title in it. For the cover art that isn't there, as soon as I connect to the network the rest of the cover art will show up.
Boxee Box 1, 2 & 3(Boxee+ v1.4.6), PCs(v22.214.171.12496) Boxee TV(v126.96.36.19981)
Server 1: HP MSS WHS (56TB) (NFS/SMB)
Server 2: unRAID (32TB) (NFS/SMB)
Server 3: unRAID (27TB) (NFS/SMB)
Server 4: TiVo Desktop (6TB RAID 5) (SMB)
NAS 1: Intel SS4200-E (4TB RAID 5) (NFS/SMB)
NAS 2: Dlink DNS-321 (1TB RAID 1) (SMB)