Have you looked into tape libraries i.e. The HP StorageWorks MSL4048. Obviously you would be looking at the uncompressed figure (144TB per unit)assuming your pictures are already compressed.
Also, hard drives are cheap, but you would need servers to manage around 200 of them and that will cost a lot to run, not to mention maintain. Tape, in my eyes, is still the best way to archive data.
At $5k per unit (first froogle result) that's only $35k for a petabyte; less than 1/3 the price of assembling your own backblaze-type storage. Not bad.
I think you've got the math backwards -- the native capacity is 38.4 TB per unit. If your data is compressable, you can store roughly 2X the amount of data in the same physical space, which is how they come up with the 76.8 TB figure.
