Last time I checked, the filechooser letted me choose a file from my gallery, I did not test in a wrapper, but really if this does not work inside crosswalk, then I would consider it as a bug.
You may want to accept only the files image/* (this is the mime type of image files, it will ask the user for a picture in his gallery, or camera), and accept multiple files (not tested multiple files myself on a phone).
Also maybe there are a filechooser plugin in cranberrygame list of cordova plugins.