How often does Google ImageBot update it's database?

Question's in the title - How often does Google ImageBot update it's database?

I see my images getting loaded into google's image search database very slowly. Anyone have any dates / figures about how often there's an image search update? "About every 6 months" seems to be a common general answer, but does anyone have anything to add? Last update was when? Historical trends?

 

Also our images too, Google only update two pic(s) in our site, too few and too slowly.

 

Google Image indexing is way too slow compared to page indexing.
But, with proper optimization, it may get a high indexing rate.

 

The reason is that Google already knows about contents to be the main factors in displaying data when people are making queries online and into its databases and so the graphical files might be of secondary important.

Images too can get indexed with proper linking and Alt text optimization.