volume shadow service can take up to 15% of the entire disk. Vista allocates, or reserves, a certain amount of hard disk space to store shadow backups. However, you the user can change the space allocated. Run cmd as administrator and type vssadmin list shadowstorage. This will tell you how much room is allocated and how much of the allocated room is being used. Then type vssadim resize shadowstorage and use the given parameters to resize accordingly.
For example:
vssadmin resize shadowstorage /On = C: / For = C: / maxsize = 3GB
The / On = C: indicates that the photos will be saved on the C drive
The / For = C: indicates that the photographs are those of the C drive
Best of Luck