Normally the command would indeed be
open -a Panini.app '/Volumes/Macintosh HD/Users/full/path/to/file.jpg'
But that would be for an application made for OS X with the special "structure" that passes command line switches "through" the bundle.
As this is not a real OS X application you need to do it the Linux way by calling the binary directly.
So that would make:
/Applications/Panini.app/Contents/MacOS/Panini '/Volumes/Macintosh HD/Users/full/path/to/file.jpg'
Change the /Applications path to the appropriate path in case you have Panini.app somewhere else.
And that works. Just tested it.