I don't think you want to be using RAID-5 for scratch disks.
A RAID-5 configuration needs to access all drives for each read/write cycle. This means that to write 100 blocks of data to a 4 drive RAID-5 setup, you need to do 100 seek/write operations to each of the 4 drives for a total of 400 operations. In RAID-5, a little part of each of those 100 blocks is stored on each one of the 4 drives, which makes it possible to reconstruct the data if one of the drives fails.
In a RAID-0 (striped) configuration, the read/write operations (I/Os) can be done in parallel. In a 4 drive stripe/100 block write example, the system will split the writes up so that each of the 4 drives gets 25 of the blocks which are written in parallel across the drives. (i.e while drive 1 is writing block 1, drive 2 is writing block 2, etc) For a given amount of time, you can get 4 times as much data read/written as in the RAID-5 setup. In RAID-0, each one of the 100 blocks is stored on only one drive, so if that drive fails, the data isn't recoverable. You gain performance but lose reliability.
For completeness, a RAID-1 (mirrored) setup gives you data reliability at reduced write performance (you need to write each block to each of the drives in the mirror), but increased read performance (you can read a block off any of the mirrors since they are identical, so you can do multiple reads in parallel).
RAID-5 is about data reliability but you trade off performance for that reliability. RAID-0 is about favoring performance over reliability. In the case of a scratch drive, you want to favor performance, not reliability, so you don't really care if the drive crashes - it was only temporary data to start with. For your image storage, you probably want to favor reliability, although you still need good backups. RAID-1 or RAID-5 will protect you against a drive failure, but not against you accidentally deleting a file. That's what backups are for.