As far as I know, it can't be done, you cannot make Lightroom the default for opening images.
Images must be imported into Lightroom in order for you to be able to use them.
Lightroom is a "Database" program - it is not designed to 'open' images, it opens database catalogs! (of the .LRCAT type)
You 'load' your images into the database catalog by doing "Import" so that Lightroom creates a reference to the image.
If anyone says they can 'open' an image with Lightroom, they are probably having the image appear in the Import dialog.
I would be best not to try and make LR a default for image files.