Hi,
this is just my opinion and i'm not sure about it, but:
- there is no archive and purge procedure for Knowledge Documents and regarding to SDM policy there is no way to purge attachments separatly keeping parent object unarchived;
- as image format like jpeg or gif are already compressed and additional archivation could give only 1-3% of size advantage, so archiving them is not necessary.
Regards,
cdtj