The company issuing the loans, Art Capitol Group, is essentially a pawn shop for very wealthy people. From the NY Times article:

Art Capital issues loans of $500,000 or more at interest rates from 6 percent to 16 percent. Fail to pay and you lose your Rubens; several of the works on display in Art Capital’s office on Madison became subject to sale after their owners defaulted.

And:

Art lenders typically lend up to 40 percent of what they appraise the artworks to be worth, and usually take possession of the works.

The prominent example in the article, photographer Annie Leibovitz, pawned the rights to her entire catalog of images for a $15.5 million loan to cover mortgages on her many properties and legal expenses. That means they valued her lifetime of work at over $38 million.