Why steal a famously unique object?
What did Franck intend to do with the painting if he had got it? Any art dealer with the funds to pay him for it would know it had recently been stolen, it was all over the news. He couldnt break it down into more easily sellable quantities as is possible with drugs, gold or jewels. Same goes for Elizabeth. Did she really go to all that trouble just to have something nice to hang on her wall?
How is art theft a viable criminal enterprise?
That aside, movie was okay. Better than Slumdog Millionaire. Not as good as 28 Days Later.